Now, it is true that, after all the predictions of interminably rising crime in America, the rate is falling (for all except the most violent crimes). This is a pleasure and a delight. But the reason for the trend is much less clear. Hence the social science conundrum.
Inevitably, the mayor of New York links the fall to his policy of zero tolerance, especially now that he faces Hillary Clinton in the local senatorial race. Some economists link the fall to the United States' amazing continued prosperity, which helps to explain President Clinton's good showing in opinion polls - notably among black Americans - come hell, high water or Monica. But some criminologists point to America's grievous penal policies, which put more and more people in jail for ever-longer terms. In Michael Howard's words, "prison works", in the sense that, when you are in jail, you can't be committing any crimes on the street.
But abortion? Professor Steve Levitt, a University of Chicago economist, and John Donahue, a Stanford University professor of law, reportedly say that, in the early Nineties, 15 per cent of the fall in crime in some parts of the United States was due to the Seventies legalisation of abortion. Poor and/ or ethnic minority young mothers, whose children might be expected to turn to crime, didn't continue their pregnancies. Years later, the potential criminals were not around to do the crimes; they had gone up in smoke in the clinic incinerator.
Before you throw up your hands, remember two things. First, this study - which no one, I think, on this side of the Atlantic, has clapped eyes on - hasn't appeared in any academic journal, and hasn't even been submitted to one, allegedly because of its controversial nature. Anyone can see that it would require some very close reasoning. Secondly, it's unclear from the reports whether tying abortion to lower crime rates may not just be a hyper-subtle (and perhaps misguided) defence of abortion, which is a far more disputatious policy in the United States than it is in Britain.
It could, after all, be a neat way to put the knee-jerk far right on the spot: which is worse - abortion or crime? I remember using a similar technique in the Eighties, to argue for reform of the drugs laws. Mrs Thatcher, I noted then, had been elected on a crime-cutting ticket. The swiftest way to bring the crime rates down, I reasoned, would be to take a large slice of the drugs offences out of the courts.
I cannot claim that the argument, however libertarian, won the day. Nor would I put any bets on the Levitt-Donahue hypothesis halting any attacks on abortion clinics. But I put these provisos in, because it's always essential to know what a study really says, not what it is reported to say.
We know this, most recently, from the skewed debate about what Dr Pusztai at Aberdeen University did, or did not, find out about genetically modified crops. In the United States especially, anything that implies that genes are more important than environment is guaranteed to cause an explosion. The Levitt-Donahue hypothesis relies on the thought that criminals are born, not made. If they are prevented from being born, then you have cut down on crime.
In Soviet Russia, the concept that everything, but everything, could be environmentally determined led to dogmatic lunacies, under Stalin and after him, which wrecked Soviet biological research. This was done in the name of all-powerful Communism. In the United States, there has generally been no such proscription of research. But hackles rise at anything that seems to deny all citizens' God-given right perpetually to make and remake themselves. On this interpretation, everyone is born as a tabula rasa, a blank sheet on which the future has still to be written.
Arthur Jensen, in the United States, ran into a hail of anger when he tried to explain the perceived failure of the Sixties Head Start schools programme in terms of a race-IQ link. So did the psychologist Hans Eysenck, when he spelt out Jensen's arguments in Britain. Admittedly, there were all kinds of problems with these studies. A generation later there would be much more agreement, based on studies of twins, that a very great deal of potential intelligence is inherited.
Arguments about any racial link would remain as contentious as ever. But this doesn't mean that Jensen and Eysenck had no right to say what they did. Arguments, however difficult, should be conducted openly and in public. They can then be stood up - or shot down. This is what tolerance means. Any fool can tolerate what he agrees with. The test is to tolerate what you dislike, or even hate. (See the Salman Rushdie row, passim.)
The problem with the Levitt-Donahue hypothesis isn't what they reportedly say. It's that the argument is being conducted in a shadowy arena, away from open debate.
You could see Levitt and Donahue's work, conceptually, as tying in with the neo-Darwinian interest in the social aspects of genetics. Before Geoff Mulgan moved into Downing Street as an adviser to the Prime Minister, the think-tank he directed, Demos, was rather keen on neo-Darwinism. But this line of thought suffers from the difficulties all ethologists have found in leaping from animal studies to conclusions about human beings. The author's chapters about animal behaviour crackle along convincingly. The trouble lies in the cross-over last chapter. The existence of human language and an alphabet - a culture - intervenes.
This was where Konrad Lorenz, the father of all such recent arguments, stumbled. So, also, did EO Wilson, the patron saint of much current neo- Darwinism, in his book Sociobiology: the new synthesis. (He had to suggest the existence of a "culture gene" to get round the difficulty.) Professor Wilson was excoriated almost as much as Jensen, when he first published. The attacks were grievously personal. But was Harvard University Press wrong to print Sociobiology? No, it was not.
Above all these debates hovers the spectre of Social Darwinism, forever associated with the name of Herbert Spencer, who invented the phrase "survival of the fittest". It is also associated with the racist arguments and actions of the eugenicists, which reached their nadir at Auschwitz. But we no longer blame, for example, Nietzsche for the rise of Hitler, even if Hitler did put Nietzschean arguments into his Mein Kampf grab- bag. And it can't be denied that the abortion-and-crime hypothesis does raise very bad memories. It would help if Levitt and Donahue achieved proper, above-board publication.
But social science is not about creating forbidden territories. It should be about expanding the argument. In thinking about the predicament of American ghetto families, consider, rather, what William Julius Wilson - a black sociologist at Harvard, who made his name at Chicago University - wrote in a paper published in 1998 by the Centre for the Analysis of Social Exclusion at LSE:
"In the eyes of employers in metropolitan Chicago, the social conditions in the ghetto render inner-city blacks less desirable as workers, and therefore many are reluctant to hire them."
This may have more impact than anything else, on the numbers turning to crime. It also has the great advantage of being something that we can hope to change.
The writer is a senior Fellow of the Institute of Community Studies