One of the longest-running controversies in history has been that between those who believe intelligence to be inherited and those who see it as determined by environment.
If time has not resolved that question, it has at least led to sharper definitions of the question and a muting of some of the dogmatism among those on both sides of this issue.
The eugenics movement of the early 20th century was based on the fear that, since people of lower mental ability tended to have more children than people of higher mental ability, the average level of the nation's intelligence would tend to decline over time.
It is hard to escape the logic of that argument. But that logic could be its undoing.
The research of Professor James R. Flynn, an American expatriate living in New Zealand, has revealed that the number of questions answered correctly on IQ tests has risen very substantially in more than a dozen nations, in just one generation.
Such a thing should not have been possible, according to the assumptions and logic of the eugenicists.
Historically, those who emphasized the role of environment in intelligence went overboard in the opposite direction.
By the end of World War II, the racial fanaticism of the Nazis had discredited the role of heredity. Some even claimed that science had proved the intellectual equality of the races.
Science had in fact proved nothing about the intellectual ability of races, one way or the other.
A landmark scholarly article in 1969 by Professor Arthur Jensen of the University of California at Berkeley exposed the weaknesses in the prevailing environmental arguments, as Professor Flynn's later research would expose the weaknesses in the heredity arguments.
Unlike others on the heredity side of the argument, Professor Jensen saw no need to dismiss environmental factors or to claim that some races were fit only to be hewers of wood and drawers of water.
One of the ironies of Jensen's landmark article was that it argued that the educational performances of children from disadvantaged groups could be greatly improved, even if there was no corresponding improvement in IQ scores.
All of that was lost in the shuffle amid the outraged reactions to Professor Jensen's challenge to the prevailing environmentalist orthodoxy.
He was denounced as a racist, and his attempts to speak on various campuses were disrupted or prevented. The net result of this mindless name-calling and hooliganism was that the heredity argument appeared to be unanswerable.