XYY: Fact or Fiction?

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XYY: Fact or Fiction?

by Dirk Elseviers

‘Science for the People’ Vol. 6, No. 5, September 1974, p. 22 – 24

You’ve just returned home from the Lying-In Hospital in Boston bearing a beautiful normal baby boy. You and the father let out a quiet sigh of relief — everything went alright, the future looks bright, true happiness is yours. In a few days, however, a message arrives from Dr. Stanley Walzer, a child psychiatrist associated with the Lying-In Hospital, informing you that your child has a chromosomal abnormality: your pride and joy has been born with XYY sex chromosomes instead of the usual XY ones. (Female sex chromosomes are XX.) What does this mean? Dr. Walzer explains that males with XYY chromosomes have been found to have behavior problems. What kind of problems? Well, violent and aggressive behavior have been found among the XYY inmates of mental-penal institutions. Since the evidence is not conclusive, Dr. Walzer would like to do a more careful study. Fear and anxiety overwhelm you. Is that a little criminal in the cradle? In your case there is no need to worry. Dr. Walzer has been funded by the National Institute of Health (NIH) to conduct a study of newborn XYY children. He assures you of psychiatric help should any sign of unusually aggressive or violent behavior be found in your boy. 

While you and the father alternate between the desire to help your baby and legitimate anger (who asked the goddam doctor to do such a study anyway?), the die is cast. Among the pile of forms you signed at the hospital one indeed states: “All male infants get a genetic screening blood test as part of a large NIH grant. This has become an integral part of the hospital’s routine and we hope soon to include all infants in this worthwhile study. If an abnormality is found, you will be informed.” Although you still have the right at this point to refuse participation in the study, you do not have much of a practical choice: you would never forgive yourself if something indeed went wrong. You agree to have Dr. Walzer follow up on your kid. 

This may sound like science fiction, but is happening here in Boston despite the fact that there is no such thing as an XYY syndrome, no evidence that extreme aggressive or violent behavior is associated with XYY chromosomes and no proof whatsoever that XYY males have criminal tendencies. 

The Birth of a Myth 

According to a National Institute of Mental Health study on the XYY chromosomal abnormality,1 the first published report of a man with XYY chromosomes appeared in 1961. Between 1961 and 1968, screening of males with some mental or physical abnormality revealed several more XYY’s. Some investigators noted that such males were unusually tall and several had histories suggesting violent and aggressive behavior. 

The controversy hit the media when defense attorneys in several murder cases started claiming that their clients were not responsible for their criminal behavior because an XYY constitution had been detected and, hence, the criminal behavior was genetically predetermined. The press picked up these arguments. And so a myth was born. In 1968, Richard Speck, convicted of murdering eight Chicago nurses, was reported in the press to have XYY chromosomes. Later this was discovered not to be so, but a retraction was not publicized. 

The Evidence 

To obtain evidence of the correlation between possession of XYY chromosomes and violent aggressive behavior, about forty studies in Europe, Australia and the United States tested inmates who were already in mental-penal institutions. Pooling these somewhat dubious data, Hook2 estimates that 2% of the inmate population are XYY. In 50% of these studies only males over six feet tall were tested. Very few studies have concerned themselves with the prevalence rates of XYY in the general population. Combining the data which are available on the “normal” population and on newborns (these rates are not necessarily the same), one can arrive at an estimate of .1 to .4%3. From these data it might appear that a larger percentage of XYY males are in such institutions than in the general population. A recent review article, however, brings even these data into question on the basis of newer studies.4 

The assumptions that (a) inmates of mental-penal institutions whether XYY or not are violent and aggressive and that (b) presence of XYY chromosomes predict criminal behavior, require examination. Nobody knows how many successful aggressive businessmen are XYY! It could well be that XYY males, compared with other “criminals” may simply be less adept at evading arrest. For example, many are very tall and easily spotted in a group of people coming in contact with the police. They might be taken for the leaders because of their height and therefore held for whatever happened. Or the severe acne supposedly associated with XYY males could have significant effects on how other people relate to them, thereby channeling them toward aggressive or criminal behavior. 

It appears that the stigma of extremely violent and aggressive behavior of XYY people derives from the simplistic view that if a single Y chromosome contributes to maleness (as against the female XX), then an extra Y chromosome will make a “super-male” having an extreme male-behavior trait, e.g., aggressiveness. Interestingly, an XYYY male has been described, who exhibited no abnormally aggressive behavior.5

What are the criteria for putting people into mental-penal institutions? What is “deviant” behavior and who defines it? What is criminal, abnormal or anti-social? In one article on the XYY syndrome, some investigators suggest homosexuality and masturbation as indicators of deviant XYY behavior.6 The lack of any real criteria concerning these aspects in the published studies raises, of course, questions as to what is the appropriate control population. The answers may be found more in the social than in the genetic conditions. 

With so many undefined variables and such slender statistical data, a scientific case for a correlation between possession of XYY chromosomes and criminal behavior cannot be established. There are also contradictory data to invalidate the relationship. For example, there is no consistent indication of violent and aggressive behavior among XYY’s. On the contrary, among all inmates of institutions the XYY’s show significantly fewer crimes against people compared with other inmates; they are also not markedly different in intelligence or hostility.7 There is, in fact, a wide range of physical and behavioral traits to be observed among all the XYY’s studied so far. Some are actually female! 

A direct causal relationship between the XYY genetic character and supposed “socially deviant” behavior has not been demonstrated. It is not clear that such a relationship could ever be demonstrated convincingly and further that the problem is worth studying. In fact, the major effects of these studies are likely to be harmful to people. 

Social-economic Background of the XYY Studies 

The wave of XYY studies is not an isolated incident of bad science but rather a reflection of a general political and social-economic climate. More and more we find that pseudo-science is being used to explain away social problems. Large psycho-surgery programs, which blame ghetto violence on brain defects, were initially funded by NIH and cut off only through public pressure. Shockley, Jensen and Herrnstein propose that black and/or lower-class people are genetically inferior, so it is a waste of time and money to provide equal opportunity in education and life.8 A theory of genetic determinism for criminal behavior comes in handy for an administration which spends more money on war (is this criminal?) than for the liberal social programs of the ’60’s. Although a correlation between genetic constitution such as XYY and criminal behavior is not a scientific fact, it gives pseudo-scientific backing for the current ideology of “blaming the victim.” 

One important question here is whether research of the XYY type benefits anybody. The work is obviously not being done to help the XYY people but to stigmatize them. Already there have been reports of abortions of XYY fetuses after detection by amniocentesis.9 In Maryland, all boys in juvenile jails are screened for XYY chromosomes without legal consent of the parents; the results are included in the boys’ criminal records.10 This raises the question of true informed consent. Parents signing the Lying-In Hospital forms should know the facts in this article. Even where they do wish to sign, the researcher has the obligation to follow the 1974 Health Education and Welfare guidelines on policies and procedures for the protection of human subjects. These detail the concept of informed consent; they require both comprehension of adequate information (and written proof of this is mandated) and autonomy of consent, i.e., the fair option not to participate.11,12

Informed consent is not being obtained at the Lying-In Hospital. Dr. Walzer’s excuse is that, in fact, he screens for as many as 30 chromosomal abnormalities and that it would be tedious to explain each of these in an information booklet. Walzer is not sure for how long he will follow up the children. What will happen to this option if his large NIH grant is suddenly cut off? Moreover, Walzer is convinced that the hospital would ask him to terminate his study if it received any kind of publicity, in spite of the “worthwhile” statement on the form. 

To offset the dangerous implications of studies such as the XYY one, some members of the Boston SESPA chapter have formed a group to plan action concerning the recently developed biochemical and genetic knowledge and technology.

>> Back to Vol. 6, No. 5 <<



  1. Report on the XYY chromosomal abnormality (Published by the National Institute for Mental Health/Center for Studies of Crime and Delinquincy; (5454 Wisconsin Ave., Chevy Chase, MD. 20015). Oct. 1970.
  2. E.B. Hook. Behavior implications of the human XYY genotype. Science 179: 139 (1973).
  3. E.B. Hook. Behavior implications of the human XYY genotype. Science 179: 139 (1973).
  4. M. Goldstein. Brain research and violent behavior. in Arch. Neurology. 30, Vol. I, p. 1, Jan. 1974.
  5. H. Hunter and R. Quaife. A 48, XYYY male: a somatic and psychiatric description. J. Med. Gen. 10: 80 (1973).
  6. L.F. Jarvik, V. Klodin and S.S. Matsuyama. Human aggression and the extra Y chromosome fact or fantasy? American Psychologist. August, 1973. p. 673.
  7. Report on the XYY chromosomal abnormality (Published by the National Institute for Mental Health/Center for Studies of Crime and Delinquincy; (5454 Wisconsin Ave., Chevy Chase, MD. 20015). Oct. 1970.
  8. See Science for the People, March, 1974. See also H. Lubs et al. Correlations between low IQ, race and variations in Q and C banding. Amer. J. Hum. Genet. 24: 47a (1973).
  9. E.B. Hook. Behavior implications of the human XYY genotype. Science 179: 139 (1973).
  10. J. Katz. Experimentation with Human Beings. The Russell Sage Foundation (1972).
  11. Federal Register 38, number 221, part II; Dept. of Health, Education and Welfare.
  12. Archive Editor’s note — the following footnote was included in the original text but was not referenced in the printed article: We have learned that this paragraph is being changed in the new information booklet now being printed by the hospital. In the new version it is more explicitly stated that the screening is part of a study and not really routine. It indicates that in case of abnormality the doctor wants to follow up the child by occasional visits to the home, but XYY is not specifically mentioned. As far as we can tell the new version would only antagonize those people who would not want to participate in any study at all, and therefore, our critique of this study still holds.