FOR MANY years some people have been searching for differences in the brains of men and women in an attempt to explain differences in behavior between the sexes.

For a while in the 19th century it was the smaller absolute brain size of females that was said to account for a supposed lesser ability among women. In the mid-1800s some researchers concentrated on the fact that men have more developed frontal lobes than women. Then attention switched to the parietal lobes, which were regarded as the seat of the intellect at that time.

Now we have neurologist Richard Restak reviewing a large number of research findings and reaching the "inescapable" conclusion that in many respects boys and girls think differently. As there are male and female reproductive systems, Restak tells us in the June 24 Outlook, so there are in effect male and female brains.

Restak contends that "many behavioral differences between men and women are based on differences in brain functioning that are biologically inherent and unlikely to be modified by cultural factors alone." "We ignore brain-sex differences," he says, "at the risk of confusing biology with sociology, and wishful thinking with scientific fact."

But if ever there was an example of wishful thinking disguised as scientific objectivity, it is in the distorted evidence and shaky logic exhibited by Restak.

Granted, Restak asserts that "we're not talking about one sex being superior or inferior to the other," and he says that his proposal to restructure the educational, social and work worlds of men and women is intended to help every person fully realize his or her own potential. But he is naive if he believes that his work will not be used by some to help justify the low numbers of women in many professions, and his proposal could not conceivably accomplish what he says he intends. Misleading research

Let us begin, then, with his misleading review of the research. If we look at children just after birth, before the sexes have been extensively exposed to differential treatment, we do not find the sex differences in sensation, perception and motor behavior that Restak suggests are inherent.

Yvonne Brackbill, a noted authority on infant development, recently completed a major review of all recent research published in the United States and England dealing with the behavior of babies from 1 to 30 days of age. She could find no sex difference in any of these important aspects of behavior. Even sleep patterns, indicators of brain activity, were not found to differ for the sexes.

Similarly, Restak's assertion that females are more sensitive to sound while males show an "early visual superiority" is based on an incomplete evaluation of the literature. In a comprehensive review of research in these areas, for one example, Eleanor Maccoby and Carol Nagly Jacklin of Stanford University concluded that "it has not been demonstrated that either sex is more "visual" or more "auditory" than the other."

It would be surprising, of course, if different treatment of boys and girls did not in time have some effect. As Restak notes, in American society girls on the average eventually achieve verbal superiority over boys, while boys eventually excel on the average in spatial ability. But he is sorely mistaken in suggesting that these differences are universal or immodifiable.

The female verbal superiority in this country which Restak attributes to "the female brain" is not found in Germany, as Michele Wittig and Anne Petersen report in their recent book, "Sex-Related Cognitive Differences." Amiah Lieblich of Hebrew University similarly has found that boys in Israel perform equal to or better than girls at all ages on measures of verbal ability.

Nor is male superiority in spatial ability found in all cultures. J. W. Berry and R. McArthur, for example, conducted separate and independent studies among Canadian Eskimos and found no sex differences in measures of spatial ability. Moreover, there is no doubt that where lower female performances in spatial ability is found among adolescents, in this country or elsewhere, it can be changed by training. Leaping to conclusions

A more serious error by Restak, though, is the way he leaps to cause-and-effect conclusions in linking behavior differences between the sexes to "biologically inherent" differences in brain functioning. It is believed, for example, that the left hemisphere of the brain plays a dominant role in language functions and that the right hemisphere is dominant for processing spatial information. Restak reports that on tests of spatial ability, boys used the right sides of their brains while girls were more likely to activate both sides. Fine.

But then he jumps to the unfounded conclusion that this difference is caused by something inherent about male and female brains, and that it is related to a detriment in performance for females. He does not even consider the fact that after boys are "trained" for years in our society to emphasize tasks involving spatial abilities, it would be surprising if their brains did not react differently to spatial tasks. Which is really the cause and which the effect?

Any weight watcher or body builder will tell you that one's biological status at any moment is a complex expression of the effect of both one's heredity and one's environment.The brain is not exempt from this principle. It is exceedingly difficult to sort out the contributions of genetics, biology and environment to the development of sex differences in hemispheric activity.

In the same way, it cannot automatically be assumed that a sex difference in one area such as the brain's hemispheric activity is linked to another area such as spatial tasks. For example, a study that recently appeared in Science, the journal of the American Association for the Advancement of Science, raises questions about the behavioral effects of sex differences in hemispheric activity that Restak cites so definitively.

In that study, psychologists Joseph Cioffi and Gilray Kandel of the Rensselaer Polytechnic Institute found that in contrast to earlier work, both boys and girls identified shapes better by touch with their left hand than with their right. Both sexes also identified words better by touch with their right hand than with their left. Girls performed no differently on these tasks than boys.

The researchers did find a difference in the hand best used to identify bigrams (two letters that do not form a word, such as XH), but both sexes nonetheless were equally accurate in identifying the bigrams. Further, some girls did better with their left hand, and some boys did better with their right. The authors concluded that any assertions about the differential organization of the brains of boys and girls must be carefully qualified. Groups vs. individuals

Perhaps the most profound and consequential mistake Restak makes, however, is in his confusion between groups and individuals, particularly as it affects his prescriptions for general changes in education and elsewhere.

When group differences are found, it is always an average comparing large numbers of people. While a small average difference can be significant to a statistician, it tells very little about individuals. As Leona E. Tyler, past president of the American Psychological Association and a noted authority on individual differences, has observed, average differences in tests "are trifling as compared with the individual differences in each sex."

Any approach that ignores individual variation is naive and harmful. It also leads to tortured semantics. In dealing with hemispheric activities, for example, Restak says that girls, on the average, are more likely to demonstrate such differences, yet he talks about "biologically inherent" characteristics of male and female brains. Does he mean to imply that the many girls within the average who perform the tasks "the way the boys do it" (on the average) have inherited "male brains"? Do the many boys who do better than many other girls on verbal measures have "female brains"?

When society bases educational, occupational and social policies on studies of group differences, there obviously are devastating consequences for individuals.

Whether one is talking about brain size in the 19th century or hemispheric activity in the 20th, then, there is no body of scientific fact proving that sex differences in behavior are caused by inherent differences in brain characteristics.The failure of science to establish this link is not surprising, given the powerful forces that have been demonstrated to shape boys and girls into fulfilling the expectations of our sex-role stereotypes.

That is not to say that both genetic and biological considerations for individuals are not important. But the strength of the contribution of genetic factors depends directly on how differently people are treated on the basis of their sex. If society treated the sexes in a totally equivalent fashion, differences between male and females would reflect inherent characteristics. We do not live in such a society.

What it comes down to is that some boys think differently from some girls. They also think differently from each other. The same holds for girls. We are beginning to understand the many factors that create this rich diversity. There is a new sophistication in much of the research that focuses on the interaction of genetic, biological and environmental factors, and the results of this work may, indeed, eventaully lead to the full realization of the potential within each person. CAPTION: Illustration, no caption, By Alice Kresse - The Washington Post