predispositions?

allrighty. so in 1965 there was a study in a Scottish institution on mentally subnormal males. apparently most of the inmates had been convicted of violent crime. they found that 7 of the 197 (3.5%) had the karyotype XYY (called 47XYY Syndrome), significantly higher than the percentage found in the general public (.1%). these XYY males were also 6 inches taller on average than XY males, but had no other specific traits to identify them.

in 1968 a study was begun in Boston to screen newborn boys for the XYY karyotype, and follow up on the boy's development. in 1974 the study was opposed by several scientists and halted on the basis that it was based on preliminary observations and could be damaging the boys, as they could be unfairly labeled as violent or of subnormal intelligence. also, the parents had not been fully informed of the nature of the study.

so what is the best way to approach this question of the effects of the XYY karyotype?

i agree that the study should have been stopped, if only because the parents had not been fully informed. and it is not fair to pre-label the boys. but this is a nature-vs-nurture question in a big way. observing the development of the XYY boys would really be the best way to ascertain the effects of the extra chromosome, and a large number of subjects would insure that there would be several different nurturing environments. obviously not all perpetrators of violent crime have an XYY karyotype, but does the chromosome cause a predisposition for that kind of behavior? if an XYY boy is raised in a caring and safe environment are they still likely to turn violent?

well it turns out that since the 70s there have been other studies on the XYY syndrome. boys born with an XYY karyotype are not particularly more aggressive or violent, but don't respond as well to a poor nurturing environment as a normal XY boy. they may also have a higher chance of having a learning disability (up to %50) but the majority of males with the XYY karyotype live their life unaware of their extra Y chromosome.