PSY307-Y-chromosome hypothesis

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The Y-Chromosome Hypothesis: “The Supermale”

Section 1 – The Y-Chromosome Hypothesis: The XYY Syndrome

The typical human being is born with 23 pairs of threadlike bodies called chromosomes. Twenty-two of these pairs of chromosomes determine our genetic traits, while the twenty-third pair, sex chromosomes X and Y, determines our gender. Males are usually born with one X and one Y chromosome, while females are usually born with two X chromosomes. However, in about every 1,000 newborn males, there is one newborn male who is likely to be born with one extra Y chromosome (Magner, Fillmore, & Jamieson, 2002). This genetic irregularity, which may possibly occur during embryological development or during the formation of the egg or sperm, is commonly known as the XYY syndrome.

Because males are known to be more aggressive than females, researchers began to suggest that an extra Y chromosome might make a male more aggressive and prone to criminal behavior. Thus, “the supermale syndrome” was born. In 1974, two geneticists, Jon Beckwith and Jonathan King, claimed that the XYY “supermale syndrome” was a myth, arguing that criminal or antisocial behavior caused by XYY males was not science, and simply social and medical folklore. Though there has been evidence that may suggest the XYY syndrome causes more aggressive behavior than the typical XY male, most studies involving the XYY syndrome are flawed, as experimenters are expected to find aggressive males. Although XYY males may score higher on aggression tests than XY or XXY males, there is always the possibility that these XYY males may display aggression due to characteristics displayed by XYY males, such as insecurities, emotional frustration, and/or intense feelings (Theilgaard, 1984, as cited by Magner et al, 2002). According to a number of studies on XYY males, individuals with the XYY syndrome have been observed to be taller than the average XY male, have severe acne, and have fairly low intelligence. Keeping these factors in mind, there is always the possibility that aggressive actions may be caused not by the XYY gene alone, but by the frustration or anxiety that may emerge when singled out because of their unique traits.

There is a good amount of scientific research that suggests the XYY syndrome is indeed related to aggression. But there is still much controversy as to whether or not the XYY syndrome causes aggression, since researchers cannot pinpoint exactly where such aggressive behavior comes from.

Section 2 – Identifying the Issue

On July 14, 1966, a man named Richard Speck broke into a Chicago townhouse, brutally murdering eight nurses within the vicinity. Nearly a year later, Speck was finally caught and put on trial. Reasoning was then given to the court saying that Speck’s actions were due to impulsive aggressive behaviors caused by the XYY syndrome; not of his own freewill. This appeared to be fairly believable, since Speck seemed to fall into the XYY category, physically and socially. He was taller than average, dealt with severe acne, and already had a criminal record at an early age. Later on, however, a geneticist by the name of Eric Engel provided evidence that Richard Speck’s chromosomal pattern was the normal XY chromosomal pattern, therefore blaming the murders on a genetic irregularity should not have been considered during the trial.

Section 3 – Application

One of the most well-known studies on the psychology of XYY males and aggression was conducted by a Danish researcher named Alice Theilgaard. She studied not only the personality traits of XYY males and XY males, but with XXY males as well. With the XY group as her control, Theilgaard performed several personality tests on each of the three groups, using such psychological tests as the Rorschach inkblot test, the Aggression Against a Person test (AAP), and the Thematic Apperception Test (TAT), in which participants would write stories that would reveal inner psychological states. After several personality tests were implemented, Theilgaard discovered that in general, XYY males score higher on aggression than XY and XXY males. She also noted that XYY males were found to have aggressive tendencies and were more aggressive than XY males; however their aggressive tendencies do not necessarily lead toward violent behavior toward others.

Although Richard Speck seemed to display both the physical and psychological traits of an XYY male, it was concluded that he was a normal XY male with extremely violent tendencies. This is not entirely unbelievable, considering that testosterone levels are high not only among XYY males, but among any XY men convicted of crimes as well (Theilgaard, 1984, as cited by Magner et al, 2002). Even if Speck was not an XYY male, earlier studies support that he could have still been as equally aggressive as XYY males found in prisons and mental institutions.

Section 4 – Columbine and the XYY Syndrome

In the case of the Columbine massacre in 1999, there is no evidence that shows the two student murderers carried an extra Y chromosome; however there is also no claim that analyses were conducted to test this hypothesis. If either or both of the students were shown to have the XYY chromosomal pattern, their acts of aggression toward the people they had killed may or may not be supported, based on their childhood background.

Some of the psychological traits that individuals with the XYY syndrome may display are low intelligence and poor social skills (Magner et al, 2002). However according to Dave Cullen, author of Columbine, Eric Harris, one of the students in charge of the murder, “read voraciously [and] got good grades if he tried”. Therefore if the psychological traits associated with XYY individuals remain consistent within all XYY individuals, the abnormal chromosomal pattern would not be a cause for Harris’ aggressive behavior.

According to Theilgaard (1984), aggression displayed by XYY individuals may also be explained by outside factors as well. The issue of height may affect an XYY individual’s aggression level because he may feel more powerful as he hovers over individuals of an average height—or he may feel the complete opposite, in which he would aggress over the frustration that he is different from others, or if his height had become a target for taunting.

The controversy over the correlation between aggression and the XYY syndrome raises the common question of nature versus nurture. “If an extra Y chromosome can lead to excessive aggression or hostility, it is possible that the single Y chromosome…may be the genetic root of “normal” aggressiveness (Jarvik, Klodin, & Matsuyama, 1973). Also, because the amount of aggression displayed in XY males varies from male to male, aggression in XYY males should be varied as well. This makes it even more difficult to relate the XYY syndrome to aggression, therefore we should not assume that the XYY syndrome has a direct or indirect relationship with aggression.


Baron, R.A., & Richardson, D.R. (1994). Human aggression. (2nd edition). New York, New York: Plenum Press.

Goleman, Daniel. (1992, September 15). New Storm Brews on Whether Crime Has Roots in Genes. The New York Times. Retrieved May 14, 2009, from

Jarvik, L. F., Klodin, V., & Matsuyama, S.S. (1973). Human Aggression and the Extra Y Chromosome [Electronic version]. American Psychologist, 28(8), 674-682.

Magner, L. N., Fillmore, R., & Jamieson, A. K. (2002). Are XYY males more prone to aggressive behavior than XY males. Science in Dispute, 1. Retrieved April 30, 2009, from

Money, J., Annecillo, C., Van Orman, B., & Borgaonkar, D.S. (1974). Cytogenetics, hormones and behavior disability: comparison of XYY and XXY syndromes. Clinical Genetics, 6, 370-382.

Noël, B., Duport, J. P., Revil, D., Dussuyer, I., & Quack, B. (1974). The XYY Syndrome: Reality or Myth? Clinical Genetics, 5, 387-394.

Toppo, Greg. (2009). 10 Years Later, The Real Story Behind Columbine. Retrieved on April 30, 2009, from:

Witkin, H. A., Mednick, S. A., Schulsinger, F., Bakkestrom, E., Christiansen, K. O., Goodenough, D. R., et al. (1976). Criminality in XYY and XXY Men. Science, 193(4253), 547-555.

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