The old saying ''Idle time is the devil's handmaiden'' holds true. At least that's the case for black young male ghetto dwellers, says W. Kip Viscusi, an economist at Duke University. Examining the results of a detailed survey of 2,358 youths in the worst poverty areas of Boston, Chicago, and Philadelphia, he found that those both out of school and out of work were much more likely to engage in crime than those either working or in school.
Moreover, he finds that many of these young men commit crime because they perceive it as more profitable than working.
Mr. Viscusi's study for the National Bureau of Economic Research is the latest look at crime by an ivory-tower economist. The study of crime was once largely the preserve of sociologists. But for about a decade, academic economists increasingly have applied their statistical tools and methodology to the examination of crime.
There is a widespread notion that the condition of the labor market, particularly the unemployment situation, is an important factor in the level of crime. A person with a full-time, high-paying job, most people would figure, is less likely to engage in crime - particularly burglary, theft, or other economic crime - than someone out of work.
However, economists could find only a moderate link between unemployment and crime - at least until the Viscusi study. Harvard University's Richard B. Freeman surveyed the research in this area for a chapter in ''Crime and Public Policy,'' a book produced earlier this year by the Institute for Contemporary Studies in San Francisco. He reported that some analyses saw crime going up with unemployment - and some other research did not.
Mr. Viscusi has employed a different kind of study - asking young men in areas with high unemployment and crime rates detailed questions about themselves and their crime activities. He found a clear linkage. Many youths saw crime as ''just another job.''
Indeed, he figures that about one-third of the crime in these ghettos arises from the opportunity people see to make better or easier money in crime than in a legitimate occupation; another third arises from the fact that so many are idle (unemployment exceeds 50 percent for black teen-agers in ghettoes); and the final third from other factors, such as drug dependence and so on.
In other words, Mr. Viscusi estimates that criminal activity could be cut by two-thirds if these youths were given well-paid jobs or went back to school.
In the ghetto survey, conducted in 1979-80, some 20 percent of the black youths aged 15 to 24 admitted to criminal activities. Because many wouldn't make such admissions to a survey taker, Mr. Viscusi estimates that actually at least 50 percent of these slum youths had committed crimes. These ranged from participting in the numbers racket and other forms of gambling to illegal drugs, mugging, shoplifting, auto theft, and burglary.
Mr. Viscusi calculates that roughly one-fourth of all income of these minority youths is from illegal activities.
Other findings of his study include:
* Those who drink, join gangs, smoke pot, or take drugs tend to have more crime income, with the effect of drugs being particularly strong. Those who take drugs are more likely to have the opportunity to sell drugs as well and earn a criminal income.
* Individuals who attend church services commit less crime. They are ''both less likely to have criminal contacts and may have the family background that leads them to place a higher value on criminal sanctions, thus decreasing their proclivity toward crime.''
Of course, the moral precepts taught at church may have an effect on behavior , too.
* Those youths on probation were more likely to be active in crime. That was also true, but less so, for those who had been in jail during the year previous to the survey.
* Married young men committed less crime than bachelors.
* School attendance, not the grade in school, tends to reduce criminal activity.
* Those youths engaged in crime earn from both their criminal and legitimate activities about $1,000 a year more than noncriminal youths. That's about one-third of the average income of the full sample.
* Some 71 of those surveyed admitted to spending time in jail in the past year. Using results of another earlier study in Washington, D.C., on the frequency of crime and jailing, Mr. Viscusi estimated the entire group surveyed committed an estimated 5,254 crimes in the previous year.
Mr. Viscusi found that almost three-fourths of the sample viewed the chance of arrest as being low. And if arrested, they rated the possibility of being convicted as also low. Even if convicted, few believed they would go to prison. Then - even if arrested, convicted, and sent to prison - most figured they would not lose their friends. ''Prison appears to have a negligible stigmatizing effect among one's peers,'' he noted. Nor did the youths figure there was much chance of losing their wives or girlfriends.
Studies by other economists have shown that crime goes down when the chances of criminals being caught go up. Mr. Viscusi agrees. So he concludes that the way to reduce ghetto-bred crime is to catch more criminals, send more to jail, and improve economic opportunities for ghetto youth. The economic recovery may also help, he adds.