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.
Flynn's research has now provided the strongest answer. The amount by which IQ test performance has improved for whole nations exceeds the IQ difference between blacks and whites in the United States or other groups in other countries.
While Flynn's work is widely known among academic specialists, it remains largely unknown to the general public.
That is an especially painful loss, not only to our understanding of the complex IQ issue, but also to our understanding of the need to be able to discuss controversial issues rationally, instead of emotionally and violently.
Last year Jim Flynn debated Charles Murray at the American Enterprise Institute in Washington. It was a model of how two decent and honorable people should rationally confront their differences over serious issues.
In his final summation, Flynn mentioned Arthur Jensen for the first time that day, saying how shameful it was that people had lashed out at Jensen instead of dealing with his arguments -- arguments that Flynn has done more to rebut than anyone else.
Jim Flynn is scheduled to return to his native United States this month, as part of an international tour to promote his latest book, "What is Intelligence?"
He is scheduled to talk at Harvard, Berkeley, the University of Chicago and other academic institutions. But thus far there is little indication that any of this will reach the public through the mass media. That is truly a shame.
We need both his knowledge and his example.