Are XYY males more prone to aggressive behavior than XY males?

Are Xyy Males More Prone To Aggressive Behavior Than Xy Males 2868
Photo by: Melissa Schalke

Viewpoint: Yes, the best studies of XYY males indicate that they are more prone to aggressive behavior than XY males.

Viewpoint: No, the presence of the extra Y chromosome in XYY males does not in and of itself produce aggressive behavior in those affected; dealing with aspects of the condition during adolescence is a more likely explanation for any later social difficulties experienced by XYY males.

The debate about the XYY karyotype can be seen as part of the old debate about nature and nurture. The belief that nature, or biological determinism and inheritance, is more important than environment and education was the basis of the field that Francis Galton (1822-1911) called "eugenics." According to Galton, talent, character, intellect, disposition, and other aspects of "natural ability," as well as physical features, such as height and eye color, are governed by heredity. Similarly, a tendency to vice, alcoholism, feeble-mindedness, and criminality are inherited. Although eugenics became a disreputable concept in the first half of the twentieth century because of its association with involuntary sterilization laws and Nazi barbarities, the essence of Galton's premises were incorporated into human sociobiology after E. O. Wilson published Sociobiology in 1975. Those who supported the hypothesis of a direct link between the XYY karyotype and aggressive, even criminal, behavior tended to favor the old concept that "biology is destiny." In this case, "biology" was manifest in the chromosome number of XYY males.

Since the 1950s clinical cytogeneticists have discovered several major syndromes in which the number of chromosomes per cell nucleus differs from 46, the normal human chromosome number. The human chromosome complement consists of 23 pairs of varying size and shape. Normally, 23 chromosomes are inherited from each parent. In each set of chromosomes, 2 are known as the sex chromosomes (X and Y), and the other 22 pairs are known as the autosomes. Human females have two X chromosomes, and males have one X and one Y chromosome. Errors in chromosome patterns can occur during the formation of the egg or sperm or during embryological development. As a group, chromosomal abnormalities contribute to reproductive loss, infertility, stillbirths, congenital malformations, mental retardation, abnormal sexual development, mental retardation, and cancers. Chromosomal abnormalities appear to cause a significant fraction of all spontaneous abortions. At least 60 syndromes of varying severity are associated with specific chromosomal abnormalities. Abnormalities in chromosome number (a condition known as aneuploidy) in humans are usually attributed to maternal origin and increase with advancing maternal age.

Segregation of the chromosomes during meiosis, the mechanism that produces sperm or egg cells, is a complex process that appears to be quite error prone in humans. About 20% of eggs and a small percentage of sperm may be aneuploid; that is, they have a numerically unbalanced set of chromosomes. Most chromosome imbalances and abnormalities are so detrimental to development that affected embryos die during the first trimester of pregnancy. The exceptions include embryos with abnormal numbers of the sex chromosomes. Apparently sex chromosome anomalies are less likely to be spontaneously aborted than aneuploidy involving the autosomes.

Abnormal numbers of the sex chromosomes seem to produce less severe clinical manifestations than aneuploidy involving the autosome. Geneticists suggest this is due to the fact that one X chromosome is normally inactivated and the Y chromosome has very few genes. Although the X and Y chromosomes are very different in size and do not exchange regions during meiosis, they do share two small regions called "pseudoautosomal regions" at which they pair during meiosis. In addition to a considerable amount of so-called junk DNA, the Y chromosome contains a sex-determining factor. Aneuploidy of the sex chromosomes is fairly common. When considered as a group, the most common aberrations of the sex chromosomes (XXX, XXY, XYY) are found in about 1 in every 500 live births.

When clinical patterns associated with abnormal numbers of sex chromosomes were discovered in the 1930s and 1940s, these conditions were named after the physicians who first described them. Thus an extra X chromosome (47, XXY) results in Klinefelter's syndrome, and the lack of one of the X chromosomes (45, X) is known as Turner's syndrome. Since the 1960s, cytogeneticists have used the chromosome constitution to name chromosomal aberrations. When the chromosome constitution 47, XYY was discovered in 1961, individuals with this chromosome pattern were called XYY males.

About 1 in 1,000 newborn males has two Y chromosomes rather than one; this is known as the 47, XYY chromosome pattern (other names: 47, XYY karyotype, 47, XYY syndrome, diplo-Y syndrome, polysomy Y, YY syndrome). The 47, XYY chromosome karyotype is the result of errors that occur in the formation of sperm cells (paternal meiotic nondisjunction) or errors that occur when the fertilized egg begins to divide (postfertilization errors). Therefore, unlike many autosomal chromosome abnormalities, the XYY type is not associated with advancing maternal age. Advancing paternal age has little or no effect either. About 80% of males with two Y chromosomes have the 47, XYY chromosome constitution. Some men diagnosed as XYY actually have a "mosaic" chromosome constitution; that is, some of their cells are 46, XY and others are 47, XYY. Sex chromosome abnormalities can be diagnosed before birth by amniocentesis and chorionic villi sampling. During the 1970s and 1980s, reports associating the XYY chromosome pattern with criminal behavior led many parents to abort fetuses diagnosed with XYY during prenatal chromosome examinations. The frequency of induced abortions of XYY fetuses decreased as further studies cast doubt on the original hypothesis.

At birth, XYY babies are usually of normal birth weight and length. Their development is generally normal during early childhood, and they enter puberty at about the same age as their XY counterparts. Some studies indicate that the IQ of XYY males is generally within the normal range, but usually lower than that of their normal siblings. However, XYY males seem to have delayed speech development, and as children and young adults they may exhibit a tendency toward behavioral and learning problems caused by distractibility and hyperactivity. XYY males do not have an increased risk of diseases as boys or adults, except for severe acne, and life expectancy is normal. As adults, XYY males are generally taller and thinner than their brothers or fathers. In other respects, their appearance and fertility are essentially normal. Indeed, the level of testosterone (male hormone) is not significantly elevated in XYY males. Sexual development, secondary sex characteristics, and fertility are essentially normal, despite somewhat lower sperm quality. Studies of the sperm produced by XYY males suggest the extra Y is usually eliminated during the formation of sperm. Thus XYY fathers rarely have sons with two Y chromosomes.

When the XYY type was discovered, some researchers speculated that the presence of an extra Y chromosome might make a male more aggressive and prone to criminal behavior. The popular press referred to this condition as the "supermale" syndrome. Some early studies of prison populations and mental institutions seemed to confirm this hypothesis. But further research raised questions about the initial concept and methodology. Later studies of the general population, and close studies of the maturation of XYY individuals, cast doubt on any direct and simple linkage between the XYY type and criminal behavior. Initial reports of a slight deficit in mental abilities eventually gave way to findings of a wide spectrum of IQ scores. Researchers have attempted to define a specific psychological profile of XYY males, but some researchers argue that no specific behavioral characteristics can be related to the XYY chromosome pattern.

Early studies of XYY males suggested they were 10 times more likely than XY men to be found in criminal populations. XXY men and XXX women were said to be more commonly found among mentally retarded or psychotic patients than XY men and XX women. Popular science writers promoted the idea that men with an extra Y chromosome were more aggressive than XY males. Some reports claimed that the prevalence of XYY men in prison was at least 25 to 60 times as high as the prevalence of XYY males in the general population. This led to the belief that XYY males are more likely to commit acts of criminal violence. Richard Speck, the killer of eight student nurses, falsely claimed he was a "victim" of XYY syndrome. Later researchers argued that sampling errors caused by conducting such studies in mental hospitals and prisons had misrepresented the effect of anomalous chromosome patterns.

The medical literature of the 1990s suggested that although 1 male in 1,000 live births is an XYY male, most go through life undiagnosed. Apparently, most do not look or behave in a way that results in testing for chromosomal abnormalities. Carefully controlled longitudinal studies of individuals with various sex chromosome anomalies conducted during the 1990s suggested that previous reports of antisocial behavior and mental disorders were misleading because of bias and sampling errors.

As early as 1974, prominent geneticists Jon Beckwith and Jonathan King called the notion of a dangerous XYY supermale syndrome a dangerous myth. This idea was primarily based on assumptions about the tendency of males to be more aggressive than females and early studies of XYY males in prisons. Beckwith and King argued that the assumption a male with an extra Y chromosome would develop antisocial or even criminal behavior was social and medical folklore, not science. Nevertheless, although researchers have generally rejected the erroneous impressions created by early studies of XYY males in prison, researchers continue to suggest that XYY males as a group exhibit an increased tendency toward aggressive behaviors. Scientists continue to debate the possibility that further research will uncover and explain subtle effects of the extra Y chromosome.


Viewpoint: Yes, the best studies of XYY males indicate that they are more prone to aggressive behavior than XY males.

The XYY syndrome is a genetic irregularity that gives the male an extra Y chromosome. All males inherit two "sex chromosomes" from their parents—an X chromosome from their mother and a Y chromosome from their father. The Y chromosome determines sex. In 1 out of about 1,000 males (XYY males), an extra Y chromosome is inherited.

XYY men (sometimes called supermales) have been of interest to psychologists and criminologists because of the suggestion that these males may be more aggressive and more prone to violence than males with a single Y chromosome. When evaluated by a battery of psychological tests, XYY males do exhibit more aggressive behaviors than XY males.

Flawed Studies

Although some studies suggest (and many researchers imply) that XYY are more violent and less intelligent than normal males and, as a result, are more often institutionalized in mental hospitals and prisons, the best exploration of XYY male aggressive behavior looks only at measurable aggressive tendencies, not at end points such as criminal acts of violence and incarceration rates.

Studies of XYY men and criminal tendencies aim at examining violent and aggressive behavior in XYY males, and most start their research in prisons and mental institutions—places where they are bound to find such behaviors, whether exhibited by XYY males or XY males. These researchers are less interested in examining aggression and more interested in understanding the roots of criminal behavior. Generally, researchers do not find a greater number of XYY males in prison, and they use those findings to suggest the XYY male is not more aggressive than normal males. This thinking confuses cause and effect with correlation.

For example, renowned criminal behaviorist S. A. Mednick concluded after research on the criminal records of Danish men that XYY men are no more likely to commit crimes of violence than XY men. Although his conclusion might be valid, many researchers have used it to suggest XYY males are no more aggressive than XY males. The data, however, should not be extended to this conclusion. Incarceration rates and types of crimes committed are data that reflect incarceration rates and type of crimes committed. They do not reflect XYY men's aggressive psychological attributes and tendencies, many of which may be noncriminal in nature.

One measure of aggressive behavior outcomes may be violence and subsequent incarceration, but aggressive behavior (if it is a trait of XYY males) may not always have violence and prison as an end point. Thus the many studies that show the percentage of XYY males incarcerated for violent crimes is not significantly greater than XY males misses the point. Many factors can intrude on aggressive behaviors so that crimes are not committed and XYY men not imprisoned.

Testing Validity

A better, more valid measure of XYY tendencies toward aggressive behavior can be examined with appropriate psychological testing employing outcomes suitable for valid testing. Validity in scientific testing is important. Validity does not mean accuracy. Validity means that a study or experiment tests what it is meant to test. Therefore, if we seek to find out if XYY men are more aggressive than XY men, aggressive behavioral tendencies—not acts of criminal violence and subsequent incarceration rates—should be the measure.

Thus when appropriate and comprehensive psychological testing is implemented, men with XYY syndrome are shown more likely to be aggressive and exhibit more aggressive behaviors than normal XY men.


Perhaps the best study of the psychology of XYY men and aggression was carried out by Danish researcher Alice Theilgaard and published in 1984 in Acta Psychiatrica Scandinavica. Theilgaard studied the personality traits of XYY men compared to normal men (XY) and men with another genetic variation, XXY, with an extra X chromosome inherited from their mothers. XXY men are reputed to be more female-like and much less aggressive than either XY males or XYY males. The most valuable aspect of Theilgaard's study with respect to the question at hand is comparing aggressiveness in XYY men with the XY male participants, called the control group here.

According to Theilgaard, the frequency of men with the XYY karyotype (the number, types, and forms of chromosomes in a cell) is about 1 in 1,000. She reported in her literature review that most studies of XYY men found them to be taller than average males. In medical terms, Theilgaard added that "no single characteristic except height … has been associated with the XYY condition." She noted that because XYY males are generally taller than average, they may exhibit behavior that is more aggressive than average.

Theilgaard also noted that studies of testosterone (the male hormone) levels have been contradictory. Some studies find that XYY testosterone levels are average; others have found them slightly higher than normal. Testosterone is thought to be associated with some typical male behaviors, and aggressiveness is one such behavior.

Some researchers, according to Theilgaard, have found that not only are XYY men taller, but they are more apt to be "clumsy" and slow witted, perhaps adding to their social frustration levels. Also at question has been if XYY men test lower on intelligence tests and whether lower intelligence has a correlation with aggressive, and perhaps violent, behaviors. Some studies, reported Theilgaard, showed that XYY men had "poor performance scores" on mental tests and showed a "lack of persistence and weak concentration power." Yet other studies showed "no overwhelming evidence to suggest that the IQ of XYY individuals is lower than average."

Personality profiles of XYY men in comparison to XY men are an important source of information. In her literature review, Theilgaard noted a 1970 study suggesting that the XYY male is "a difficult child, has problems in school, shows excessive daydreaming, is a loner, a drifter, presents unrealistic future expectations, and manifests impulsiveness with sporadic, sudden violence and aggression." Note that Theilgaard's literature review of relevant studies did not find research suggesting that XYY males were more often in prison than other males. Thus she sought to answer whether XYY men might be more aggressive than XY men. Consequently, in her study she attempted to document psychological characteristics to see if aggressiveness was a common trait among XYY men. To do so, she employed standard and appropriate psychological tests.

Among the tests she used were the TAT (Thematic Apperception Test), in which study participants write stories that reveal inner psychological states; tests that use word associations to indicate psychology; and other psychological profiling examinations to evaluate XYY men's potential for behaviors and ways of thinking.

In her study, Theilgaard tested both XYY and XXY men and XY controls. Her results after examining XYY males had many implications regarding the tendency toward aggressiveness and—more importantly—many of the psychological tendencies that may lead to aggressive behaviors.

For example, Theilgaard found that XYY men "give more aggressive and less anti-aggressive content in their TAT stories compared to their controls." She also found that XYY men are more "rigid" in their thinking. Generally, she found that on all tests XYY men score higher on "aggression" than their controls. Too, the "habitual mood" of XYY men is "judged to be more low and pessimistic" than that of controls. Also, XYY men show more "evasiveness" and report more "negative relations" with people. They live lives that are "disharmonious" with more "unstable feelings" than controls.

In addition, Theilgaard found that the quality of a XYY man's childhood was judged "less positive" by the test interviewers as well as by the XYY men themselves. She also found that XYY men get along less well with their "partners" and act more aggressively toward them. Theilgaard found that XYY men were more restless and impulsive than XY men, that they were more prone to "sporadic outbursts" with more immature, intense feelings. She says the typical XYY male appears "disharmoniously" integrated with an "unfinished personality." He seems "insecure, emotionally frustrated and perhaps a little blunt in his reactions toward others," concluded Theilgaard. Once more, the XYY male is more "rigid" in thinking and may find it harder to control himself. It may be that all of these tendencies result in more aggressive behaviors.

In terms of criminal behavior, she found that XYY men are arrested more frequently than controls, but their conviction rates were not higher. However, in the test of Aggression Against a Person (AAP), she found that XYY men show less "antiaggression" behavior and more often report being a "bully." The XYY group showed a "shorter reaction time on words with aggressive content," said Theilgaard, "and provided more aggressive content in the Rorschach (inkblot) test," a test where they are asked to describe their reaction to a shape made by an inkblot. "Positive correlations are seen (in the XYY group) between reported aggression against persons and aggressive content in TAT," wrote Theilgaard.

Why XYY males might have higher levels of aggression is unclear, however. Theilgaard reported that her study found higher levels of testosterone, the male hormone, in the XYY men she studied. Interestingly, she found that testosterone levels were also high among XY men who had been convicted of crimes.


When looking at her conclusions with regard to aggression and comparing the literature of other researchers, Theilgaard wrote, "In the literature the richly faceted concept of aggression (applied to XYY men) has often been represented as a unitary idea, without attention being paid to behavioral, emotional, motivational or arousal aspects, and the emphasis has been on the destructive features while the positive side has been ignored."

She added that the word "aggression" comes from the Greek word ad-gerdios, meaning "approaching" and suggested the word be evaluated in its positive and negative aspects, which means adding achievement, leadership, dominance, and assertiveness to the more negative attributes of hostility and violence. She concludes that "consensus" on what aggression means has been hard to reach.

"This study," she concluded, "sought to let the unitarian concepts give way to more elaborated behavior constructs." What Theilgaard meant was that although she looked at XYY males and concluded they were, in fact, more prone to aggression than normal XY males, their aggressive tendencies did not necessarily end in violence, prison, or other forms of institutionalization.

"The complexity of the matter requires that the question of aggression is not viewed in isolation," wrote Theilgaard. That men with the XYY chromosome are more aggressive than normal XY men is supported by Theilgaard's psychological testing. But, in keeping with Thielgaard's request, we must recognize that aggression has many forms, with both positive and negative outcomes.

Although many studies have concluded that XYY men are not more aggressive than XY men, most of these studies have looked at prison and mental institution populations and have identified XYY men in these settings at only a slightly greater rate than would be expected. These studies may be flawed because they are self-fulfilling: they look at prison populations for aggressive and violent men and—not surprisingly—find them. That there are a few more XYY men among these populations is not too surprising, given some of the XYY psychological characteristics Theilgaard identified.

Data that reveals not greatly inflated rates of XYY men in prison can be used to suggest that XYY men are not more aggressive than XY men, but this conclusion does not account for the great variety of aggressive behaviors in XYY men that may incarcerate them. As Theilgaard noted, "aggressiveness" does not necessarily translate into violence and criminal behavior.

When she tested personality rather than incarceration rates, Theilgaard found that yes, XYY males have aggressive tendencies, that the XYY male is more aggressive than XY males, but their aggressive tendencies do not necessarily manifest in violent behavior toward other people.


Viewpoint: No, the presence of the extra Y chromosome in XYY males does not in and of itself produce aggressive behavior in those affected; dealing with aspects of the condition during adolescence is a more likely explanation for any later social difficulties experienced by XYY males.

Reports of a biological basis for antisocial or criminal behavior always raise considerable interest among both the public and the scientific community. There are several reasons for this. First, criminals and their defenders hope to gain acquittal or reduced sentences by pleading mitigating circumstances, suggesting the defendant was not responsible for his or her actions. Second, there may be a wish to absolve society in general of the responsibility for creating criminals or a strong need to believe that humans are basically good and antisocial or criminal behavior is caused by influences beyond our control. And finally we may also hope that if a biological or physical basis for criminal behavior is found, there may one day be a cure for it. Although the idea of a biological basis for criminal behavior is not new, stretching back at least to Francis Galton's theory of eugenics in the mid-1800s, the suggestion of a link between the XYY genotype and aggressive behavior was the first to provide a chromosomal demonstration.

XYY Syndrome

Also known as polysomy Y, XYY syndrome occurs in males who, instead of the usual 46 chromosomes (including one X and one Y chromosome), have 47, due to the presence of an extra Y chromosome. This type of chromosomal anomaly is known as aneuploidy of the sex chromosomes. Such aneuploidies, which can involve either the X or Y chromosome and a varying number of extra chromosomes, are the most common type of chromosomal disorders, occurring in around 1 in 500 live births. Aneuploidies of the sex chromosomes, especially those involving the Y chromosome, generally show less serious clinical symptoms than do abnormalities of autosomes, because of the restricted and specialized functions of the sex chromosomes. However, some studies suggest that the loss of balance of the gene products of the sex chromosomes can cause an increased susceptibility to other genetic or environmental factors. This susceptibility can lead to behavioral problems or the beginning of mental disorders in childhood.

XYY syndrome is not always visible in the phenotype. Affected individuals may show no signs of the condition and never be detected, unless their chromosomes are assessed or karyotyped for another, unrelated reason. However, there are some characteristics that are often found in XYY males. They are frequently very tall, compared with both the general population and other people in their own family. Over 40% suffer from a very severe form of acne in adolescence. Skeletal abnormalities are sometimes found in XYY males, but these are not normally visible and have little or no impact on the affected person. Intelligence is often lower than average, although this is not inevitable and some XYY males have a normal to very high IQ. Some affected individuals have learning problems, such as delayed speech and difficulty with articulation. EEG traces show significantly slower background activity than those of similar aged nonaffected males. This is normally a characteristic of earlier stages of development, suggesting a developmental lag in XYY individuals.

In the general population, the incidence of XYY males is around 1 in 1,000. Among tall males, this rises to 1 in 325 and among tall males in prison as high as 1 in 5, according to some studies.


The first recorded case of an XYY male was reported by Avery Sandford and colleagues in 1961 at the Roswell Park Memorial Institute in Buffalo, New York. The individual concerned was karyotyped because his wife had given birth to a Down syndrome child, and investigators were looking for any chromosomal abnormalities.

Initially, it was suggested that, since the Y chromosome defines the male, and males are more aggressive than females, a double complement of Y would increase aggressiveness. To test this idea, in 1965 Patricia Jacobs and colleagues conducted a study of inmates of a Scottish maximum security hospital. The subjects of this study were defined as "mentally subnormal male patients with dangerous, violent, or criminal propensities." Jacobs found that 1 in 28 of these subjects were XYY. This high rate, compared with the incidence in the general population, seemed to suggest that the XYY condition could indeed be linked to increased violent or antisocial behavior. Studies of other similar populations confirmed Jacobs's findings. These results sparked fierce debate, especially in the United States, fueled by sensationalist reports in the media. A large number of professional people, committed to the idea that the environment is responsible for human development, were unable to accept the idea of a genetic component to human behavior, to the extent that individual researchers were harassed and, ultimately, research in the field was curtailed.

A later study by D. A. Price and A. J. Whatmore (1967) found a significant difference in the age of first criminal conviction for XYY males (age 13, compared with age 18 for controls). The XYY group had a much higher number of convictions for crimes against property (an average of 9.0 for XYY males as opposed to 2.6 for normal XY males) but a much lower number of convictions for crimes against the person (0.9 for XYY males and 7.7 for XY males). Criminality was much higher among the siblings of the XY males than the XYY males, suggesting the XYY condition overrides familial or environmental influences. The families of the XYY males also reported a history of behavioral problems from early childhood. However, these reports were given with hindsight and may not be entirely reliable.

A number of issues cast doubt on the reliability of such studies. Many used very small sample groups, with inadequate or mismatched control groups. Some were even based on a single case of extreme antisocial behavior. Many investigators drew their samples from groups considered likely to have a high proportion of XYY men—for example, tall institutionalized men—rather than a randomized group. Finally, the investigating psychologists who were assessing the personalities of the subjects often knew whether or not they were XYY at the time of assessment, which could have led to bias.


In 1976 a definitive study was done by Herman A. Witkin, S. A. Mednick, and Fini Schulsinger, and others. Using the comprehensive records of the Danish draft board, which included tests for intellectual function, educational history, socioeconomic background, and criminal history, Witkin was able to screen a cohort of over 4,000 men. The original sample was in excess of 28,000 individuals, but due to the time and expense involved in karyotyping, only the tallest 16% of the population were screened. The sample contained 12 XYY men (1 in 345), a considerably higher proportion than would be expected in the general population.

Witkin tested three possible hypotheses to explain the previously recorded data on increased criminality in XYY males. First was the aggression hypothesis, which proposed that an extra Y chromosome would cause increased aggression and hence increased criminal activity. The second hypothesis was that the increased height of XYY males would cause them to be perceived as more threatening, and so more likely to be suspected of crime and also more successful in aggressive encounters because of their greater size. The final hypothesis, known as the intellectual dysfunction hypothesis, was that the XYY condition causes decreased intelligence, which in turn leads to more antisocial behavior, plus an increased risk of being caught.

The results of careful analysis of the data showed that the XYY males were convicted of significantly more crimes than the controls. In fact, 42% of XYY males had criminal records compared with 9% of normal males. However, these were mainly nonviolent crimes such as vehicular offenses and shoplifting. For violent crimes against people, no difference was found between the XYY males and the controls. This does not support the aggression hypothesis.

Individuals in the XYY group were significantly taller than the control males, but among the controls, noncriminals were taller than criminals. The height hypothesis was therefore disregarded.

Both mean IQ and educational attainment were significantly lower in the XYY group than in the control males. In addition, among the controls, those with criminal records had a significantly lower IQ than those without. Both these findings support the intellectual dysfunction hypothesis.

Allowing for background variables—that is, comparing XYY males with XY males of similar IQ, height, and social status—the rate of criminality in XYY males is significantly higher than for normal males, although the difference is slight. The conclusion is that XYY males are more prone to criminal behavior, but only in crimes against property and not against the person. This elevated crime conviction rate may be mediated through reduced intellectual capacity.

Much other evidence indicates that an XYY constitution does not lead to aggressive behavior. In institutions, XYY males tend to show less aggression and better adjustment than normal males. Some studies have confirmed that XYY criminals commit fewer violent crimes than the mean for the general criminal population.

Much more recently, a study of over 31,000 men found no strong correlation between the presence of an extra Y chromosome and criminality (Mednick, 1988). In 1999 M. J. Gotz, E. C. Johnstone, and S. G. Ratcliffe studied a sample of over 34,000 infants screened at birth, comparing XYY males with matched controls. Gotz also found a significantly higher frequency of antisocial behavior in the XYY males in both adolescence and adulthood. The XYY males also had a higher rate of criminal convictions—again, of crimes against property rather than people. Complex analysis of the data shows that this effect is a result of lower intelligence.

What are needed are long-term studies of the development of XYY males from birth onward, with matched XY controls for comparison. However, such studies can be problematic. In the past, a prenatal diagnosis of XYY syndrome led parents to terminate a pregnancy. Fortunately, this rarely occurs today because the theory of increased aggressiveness has fallen from favor. Another problem lies in whether the parents should be told of the diagnosis. If families are aware of the karyotype of the child, they may treat him differently and confound the study. Not to inform parents of the karyotype raises a moral dilemma, since some researchers have shown that awareness of the diagnosis and sustained counseling and advice for families have been helpful in dealing with behavioral problems in XYY boys.


Overall, the evidence suggests that XYY males may have certain behavioral and conduct problems, especially in adolescence. Such problems may be compounded by the individuals' unusual height and, in many cases, severe acne, both of which can cause acute distress to an adolescent. XYY males can show hyperactivity, impulsive behavior, and lack of emotional control. They often have a history of outbursts of temper when frustrated and crave immediate gratification. These characteristics, when combined with a lowered intellectual function and the accompanying lack of educational attainment, can lead to increased criminality. However, the crimes tend to be against property and rarely against people. XYY males can be difficult, but they are not generally dangerous. Awareness and appropriate therapy or counseling for both patients and families can overcome the problems and allow these men to lead normal lives. No sound evidence suggests that affected men are more prone to aggression or violence or that they should be absolved of responsibility for their actions, criminal or otherwise.

A prenatal or childhood diagnosis of XYY syndrome need not be cause for alarm. Many such children develop perfectly normally, with no ill effects from their condition. For some, there may be an increased risk of behavioral problems, but environment and upbringing likely will play a greater role in determining how much the individual is affected.


Further Reading

Beck, W. S., K. F. Liem, and G. G. Simpson. Life: An Introduction to Biology. New York: HarperCollins, 1991.

Ciba Foundation Symposium 194. Genetics of Criminal and Antisocial Behavior. New York: John Wiley, 1995.

Court Brown, W. M. "Males with an XYY SexChromosome Complement." Journal of Medical Genetics 5 (1968): 341-59.

Gotz, M. J., et al. "Criminality and AntisocialBehavior in Unselected Men with Sex Chromosome Abnormalities." Psychological Medicine 29 (July 1999): 953-62.

Jacobs, P. A. "The William Allan MemorialAward Address: Human Population Cytogenetics: The First Twenty-Five Years." American Journal of Human Genetics 34 (1982): 689-98.

——, et al. "Aggressive Behavior, MentalSub-Normality and the XYY Male." Nature 208 (December 1965): 1351-2.

Mednick, Sarnoff A., and Karl O. Christianson. Biosocial Bases of Criminal Behavior New York: Gardner Press, 1979.

——, et al. Explaining Criminal Behavior. New York: E. J. Brill Press, 1988.

Pallone, Nathaniel J., and James J. Hennessy. Tinder-Box Criminal Aggression. New Brunswick, N.J.: Transaction, 1998.

Price, W. H., and P. B. Whatmore. "BehaviorDisorders and Patterns of Crime among XYY Males Identified at a Maximum-Security Hospital." British Medical Bulletin 1 (1967): 533-6.

Science 289 (28 July 2000). Special edition on violence.

Theilgaard, Alice. "A Psychological Study of thePersonalities of XYY and XXY Men." Acta Psychiatrica Scandinavica 69 (Suppl. 315) (1984).

Walzer, S., et al. "The XYY Genotype." Annual Review of Medicine 29 (1978): 563-70.

Witkin, H. A., et al. "Criminality in XYY andXXY Men." Science 193 (August 1976): 547-55.



Condition where a copy or copies of a chromosome are lost or gained so the total number is more or less than the normal 46.


The chromosomes that are not sex chromosomes (i.e., those that are the same in both sexes).


Structure composed of DNA (deoxyribonucleic acid) that conveys genetic information.


Chromosomal constitution of an individual as seen in the nucleus of a somatic cell. For a normal human male the karyotype would be 46, XY (i.e., 46 chromosomes in total, including one X and one Y chromosome).


Physical characteristics of an organism.Often not identical to the genotype because organisms can carry genes that are not expressed.


Psychological test in which participants explain what they see in an inkblot.


Written tests, verbal stories, and picture interpretation.

User Contributions:

Shaylene Van Gorkum
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Nov 7, 2009 @ 1:13 pm
I have recently found out the problems we have been having with our seven year old son is he has an extra Y in his DNA. He had been on ADHD medication, that had increased his anger. He is off all medications now and seems to have a normal anger level. Is there anything natural I can give him to calm down. My husband and I ( married 11yrs.) are VERY exhausted. Jeffrey also has not slept through a night in his life I included. The sleep aid he is on ( Clonidine .1mg) helps but his body and mind are still active moves while sleeping and wakes up with night terrors, and comes to get me two or three times a night. We have lost our home and are struggling on one income. We have moved so Jeffrey could go to a school for learning disability, but they couldn't control his behavior. We are willing to do and have done anything for our son. If there is anything you can suggest, or if there is financial help please let me know, or if I can help you in any way let me know. Thank You
Michelle Kelleher
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Jan 31, 2010 @ 9:09 am
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Feb 21, 2010 @ 11:11 am
My son is 12yrs old and has been diagnoised with autism 6 mths ago then 12 mths ago with xyy can you tell me is there a connection with both diagnoises thankyou.
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Mar 24, 2010 @ 5:17 pm
I have been reading up on this subject for the last 2 years and every article is inconclusive. Nobody knows anything except that the child will be taller than you expect him to be. All research into intelligence, criminality and tendancies to violence is unfounded because the research pools are too small and are generally biased towards the area of research. The social background of the subjects is never accounted for and all research always contains the words 'can' or 'could' or something just as vague. The other common phrase is "this is normal is any range of siblings". So far, my child is no different to his class mates, if not better as he understands two languages, in his development and he is also one of the youngest in the year group. When the doctor told us of the condition before birth we knew more than he did from reading online. My advice: don´t worry too much. If you bring him up right then he'll be fine.

Michelle: It's a condition, not a disease. There's no cure.

Sue: I don't think there is any link between the conditions. None of the articles I have read have mentioned such a connection.
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Sep 19, 2010 @ 2:14 pm
how abnormal chromosomal constitution effects the human behaviour? what are the reasons,plz explain in detail.
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Sep 19, 2010 @ 2:14 pm
how abnormal chromosomal constitution effects the human behaviour? what are the reasons,plz explain in detail.
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Oct 21, 2011 @ 5:05 am
hi, I have a 5 years old son with xyy and he is ok, even more clever than my older son - 9 years old who has normal chromosal costitution. I love both my sons, don´t mind which is cleverer than other. All I want to say, that xyy IS NOT a PROBLEM.
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Feb 8, 2012 @ 4:16 pm
Hi everyone i have a 2 year old. Who as xyy syndrome. We found out when he was born and i'm trying to find out all i can about it. if anyone can help it wound be great thank you.
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May 31, 2012 @ 3:03 am
My son has Agenesis of the Corpus Callosum and was also diagnosed with a XYY syndrome. I live in South Africa but at this stage I am in the dark as to what the cause could be as well as what to expect in future. My son is now 2 and a half but still cannot walk or talk. His mother now takes care of him on a permanent basis. I also would like to know whether this could have been detected during pregnancy.
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Jul 19, 2012 @ 9:21 pm
Stefan, studies explain that xyy children show a slower development concerning speech, i am not sure about the walking later though. Also it is important that xyy sydrome is considered as a possible "predisposition" for agressivity or behavioural problems, which means that it is a genetic predisposion (or possibility), other factors such as environment and upbringing or "cobditioning" especially during adolescent years are primordial in the actual development of these types of dificulties. It is important to provide a coherent and sainly disciplined upbringing :) (as with any child)

Oh, don't forget that agressivity is not necessarily negative, there is also positive agressivity so it is not necessary to eliminate agressive tendancies but is important that guidance is present...
Randy the Atheist
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Feb 12, 2013 @ 12:12 pm
The aggressiveness of XYY males may stem from the simple fact that bigger males in schools tend to be bullies and often carries over into adulthood. This occurs regardless in XYY or XY males and thus, the correlation of XYY and aggressiveness may be flawed. In other words, the aggressive behavior may be a social effect - not a genetic one.
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Feb 4, 2015 @ 5:17 pm
It is so sad that we have to have people with these problems out in the world. Nobody should have disabilities.
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Dec 20, 2017 @ 11:23 pm
I am a 59 year old XYY. Diagnosed when i was 14. I am not a criminal, have never been arrested, have had maybe 2 speeding tickets. Yes, I am very much taller than my parents or siblings (2 sisters both XX). Our Father taught us all how to read before Kindergarten, so we were all reading before they taught us how to in second grade.I read everything I could get my hands oin, all the great classics my parents had as well as the World Book Encyclopedia. I was also doing math problems, way ahead of all my peers, and still do them for fun in my head, I did have some aggressive tendencies when an adolescent, I am not thin, never had learning difficulties in school (actually I worked through all the problems and questions in the school books and would hand them in and get extra credit for doing all the questions). As a larger than average person, I would get picked on and taunted at school, so being slightly more aggressive, could be learned, and not due to the XYY Syndrome, since when you finally have had enough taunts, etc, you may lash out - but anyone would do that. Yes, i was bored in high school, yes, I would daydream, usually because I knew the answers to all the questions. I have never had any children, probably because my parents gave me weekly IM injections of Depo-Provera to help curb my aggressive tendencies, not realizing that medicine would later be used as a birth control method. I was bored in college, didnt really have anything I wanted to do, so I dropped out, went to London and Scotland for a year, came back got my Associate of Arts, just to get something higher than the HS Diploma. I turned out OK, if I do say so myself, so give your XYY sons love and affection, extra help if they need it, but treat them like any of your other kids. They will turn out OK. (I did quit my well-paying job at a hospital 5 years running to run off and play at a music and dance camp for adults, they hired me back each time) Please keep the XYY baby and treat them just the same.
Miguel Lahunken
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Aug 25, 2018 @ 4:16 pm
An anonymous Double Y, Cro Magnon male, was given a bag of preserved cherries from Red China. The preservative in the cherries obviously contained a chemical that blocks the enzymes that change uric acid into urea in Cro Magnon people, for, this anonymous Double Y woke up with uric acid kidney stones the next morning, after eating these cherries the night before. He had never had kidney stones before in his life, and, none of his family ever had kidney stones. The stones were so bad that the stress knocked out his immune system, and he got an extremely high fever with the resultant pneumonia. It is obvious that in Red China such Cro Magnon people are left to die, while this preservative is harmless to Homo Sapiens. Also, only Homo Sapiens can ingest caffeine safely. In all other creatures, including Cro Magnons, caffeine acts like strychnine. Red China is ruthless, canning the flesh of aborted fetuses for food, and stealing the organs of the "unapproved" for transplantation. But these kind of things can happen anywhere in the world. We just have blatant proof that it happens in Red China.
Ellie Slater
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Mar 18, 2020 @ 2:14 pm
I'm trying to OSCOLA reference this and wondering if anybody knew the date that this was published as i'm including this information into my essay for my University Assignment.

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