Those who place a lot of emphasis on genetics in explaining behavior, including differences between societies, have trouble with large changes in a society over a short period of time. “The Sixties” is such a period of time. Wilson & Herrnstein give it their best shot in their History and Culture chapter in “Crime and Human Nature”. They suggest that an increasing proportion of infants with low birth weight survive into adolescence, adding that one-fourth of low birth-weight babies have a major handicap like low IQ. The number of babies with physical and mental defects did in fact increase markedly, which may also be the result of prenatal use of alcohol/tobacco/drugs or exposure to more environmental toxins (the lead-caused-crime theory). Additionally, they note that the ratio of aggravated assaults to homicides increased from about nine to one in 1935 to over twenty-eight to one in 1980, as society has become better at saving the lives of such injured persons. Since the victim in such assaults is often violent and may be as much to blame as his (and it usually is a him) assailant, this serves to increase the life expectancy of violent persons.
This is not to say that the authors put a great weight on those explanations. They regard the increasing portion of the population that is young and the urbanization of the country as the most definite factors (accounting for about 46% of the increase). And they list a bunch of other explanations without committing to them.