FEW DEVELOPMENTS in modern science have held such potential for public controversy as the recent discovery of physical differences between men's and women's brains.

Since the postwar era, students of human behavior generally shunned theories ascribing sex-related variations in intellect, perception or emotion to biological differences. Social concerns of the '60s favored explanations based on environment; feminist concerns of the '70s stressed sociological influences. By the mid-'70s, however, biological determinism began to regain intellectual respectability, raising anew the question of whether "nature" or "nurture" is dominant in shaping the self. Since then it has flourished, bolstered by two parallel developments: the discovery that each side or hemisphere of the brain has specialized functions; and the revelation of clear structural differences between male and female brains. {See box}

The resulting stir extended from learned journals to drugstore paperback racks. And though the latest studies have uncovered fascinating aspects of sex-linked cognitive and behavioral differences, to date the research has proven more exciting for what it promises that for what it actually explains about why men and women act the way they do.

The Divided Mind

The current "nature" theory builds on the discovery that humans have, not a single unitary brain, but a complex mental system built on the interaction of two hemispheres. The left (which controls the body's right side), appears to specialize in linear, linguistic, analytic, rational, deductive tasks; the right in holistic, spatial, intuitive processes.

Studies consistently show that when male and female brains are exposed to the same stimulus, they respond to it somewhat differently. It's a question of "lateralization" -- that is, of which hemisphere acts more efficiently in each case. In many kinds of linguistic and spatial tests, it appears that the same problem take a different route through male or female brains. (Of course, the differences within each sex dwarf the differences between the sexes, as they do in most behavioral studies. But the variations appear undeniable, at least in most Western studies; they do not show up reliably in all cultures.)

The written word, for example, presents two routes to meaning: the look and the sound of the letters on the page. Research results indicate that men and women process them differently. When subjects were asked to eliminate each h from written passages, males got more of the silents than did females. Researchers believe that indicates a more visual approach.

Subjects were also asked to decide which of various strings of letters constituted English words. Both sexes rejected obvious imposters more quickly than nonwords that seemed to be plausible English, but the women took significantly longer than the men. Why? Because, argue British psychologist Max Coltheart and colleagues, the men tended to begin by treating all the strings by sight -- as groups of meaningful shapes to compare with words stored in memory. Women more often began the analysis with sound, a less efficient method that adds a step to the process.

Another experiment highlighted the different strategies even more clearly. On two tests of spatial reasoning, women scored lower because they more often framed and tried to solve the problem in words; whereas men more often used purely spatial methods in a wordless mind's eye. But, the researchers emphasize, the difference was relative, not absolute: "Both sexes appear to use spatial strategies less effectively and fall back on less organized, less efficient, more concrete methods." Women, however, did so more often.

It thus seems easy to conclude that women are more dependent on right-brain functions. But the overwhelming weight of recent evidence finds women less lateralized than men. That is, they do more spatial and verbal tasks in both hemispheres -- perhaps because of cultural influences, or because they have more, or different, interhemispheric connections. (See box.)

Cells and Songbirds

Why the two sexes should show different lateralization remains a mystery. In fact, it is unclear there is any laterality at all. Our two lungs don't breathe differently, nor do our kidneys specialize in different types of impurities. A difference so apparently profound suggests an ancient origin.

Canadian psychiatrist Pierre Flor-Henry locates it at life's first emergence into multicelled, mobile creatures. What caused us to evolve from little balls of protoplasm to shapes with well-defined tops and bottoms, fronts and backs? Gravity, he says, "provided the first lever" for differentiating up from down, forward from backward, and "hence, the right and left. However, at a more fundamental level the organization of all living material -- such as the amino acids composing our very genes -- is asymmetrical."

The first definitive finding of true sexual dimorphism (different forms within the same species) in brain structure derives from neurobiologist Fernando Nottebohm's celebrated studies of songbirds. Amorous males sing; most females do not. By anatomical studies, Nottebohm determinied that singing is a function of the male's left brain. It is also a function of a gender role: A bird does not sing to brighten our springtime, but to assure the reproductive success of his own. His song announces his location to his potential or actual mate, and the extent of his territory to male competitors.

Commenting on Nottebohm's findings, Flor-Henry argues that if song is linked both to sex and to a specific hemiphere, then it follows that "in those species where the male actively seeks the female, the former would exhibit a greater degree of lateralization than the latter." Supposing that humans are descended from a line of creatures whose males hunted more actively for mates -- a proposition at present better supported by theory than by evidence -- then a male spatial superiority would be a major selective advantage for many species.

The human brain, of course, is far more complex, shifting with lightning speed among different parts of both hemispheres. Even so, Flor-Henry argues, we carry remnants of our remote sexual past. His psychiatric practice convinces him that "the differential hemispheric organization of the male and female" put them at risk for different mental illnesses, usually triggered in the weaker hemisphere. Males seem to succumb more readily to left-side malfunctions like autism, schizophrenia and psychopathy; females to mood malfunctions of the right.

But the central question remains: Why should men and women approach the same problem differently? Some argue for pure habit. Suppose that infant girls -- who will mature two years earlier than boys born the same day -- emerge from the womb ever so slightly more attuned to the human world of language. And if they pay just slightly more attention to their hearing and boys slightly more to their sight -- if girls respond slightly more eagerly to faces and boys to objects -- those sensitivities might grow into preferences and the preferences into habits of mind. Psychologist Julia Sherman believes that girls arrive with a slight edge in language ability, and that this "bends the twig toward female preference for verbal approaches to problem solution. This bent is then increased by the verbal emphasis of the educational system and by aspects of sex roles that do not encourage girls' development of visual spatial skills."

The Hormone Gambit

Why would baby girls become more verbal and boys more visual? Ever since the discovery of hormone receptors in the brain, attention has centered on the possible influences of steroids. As psychologists Diane McGuinnes and Karl Pribram point out, "receptor sites for sex hormones are concentrated largely in . . . the forebrain, focus for the brain systems that control arousal." The same system that controls attention also affects intensity of perception, they believe. "Possible sex differences might therefore reflect differences not in inherent abilities but in this intensive dimension of experience." If the signals coming over one channel -- sight, for example -- were simply more vivid than those coming over another -- such as sound -- an infant would naturally pay them more attention.

Similarly, data gathered by brain researcher Jerre Levy suggest that in boys, right-brain functions mature first. "Fetal sex steroids," she writes, "may play a critical role in determining relative maturational rates of the two half-brains and possibly of other bodily regions as well."

Another possibility has attracted considerable attention lately. It may be that a single, mysterious factor ties together migraine headaches, autoimmune diseases, dyslexia, allergies, stuttering, thyroid problems and left-handedness. Until a casual comment in 1980, no one imagined that so unlikely a catalogue shared anything in common. But then the late Norman Geschwind, a celebrated brain scientist, urged dyslexia researchers not simply to track that single disability up and down their young patients' family trees, but to look for any other genetic anomalies. When they did, they found that the most common was a single suggestive trait: left-handedness. Southpaws suffered learning disabilities at 10 times the rate of righties, and immune-system difficulties more than twice as often.

Why should that be? Suppose, Geschwind and his colleagues theorized, that a single factor retarded the maturation of both the thymus gland (vital to the immune system) and the brain's left hemisphere (responsible for both language ability and the usually dominant right hand). And suppose this same factor could account for the fact that the greatest share of dyslexics are male. This mysterious factor, of course, was none other than that jack of all physiological trades, testosterone. Precisely why the male-making hormone should have this effect -- if indeed it does -- remains one of the many mysteries that still envelop our understanding of the brain.

So what does this tangle of surmises add up to? A good deal, of course, but probably less than the breathless reporting in the popular press would have us believe. Especially since many prominent brain researchers, biologists and psychologists still challenge the entire notion of biological determinism.

Moreover, our knowledge of the brain is far from complete and changes almost daily. In June 1984, for example, a presitigious research laboratory, for years in the vanguard of its field, suddenly announced a finding that, in the words of the usually sobersided journal Science, was "so unexpected, so incredible, that they literally do not know how to explain it."

Two months earlier Nottebohm, the pioneering student of bird brains, had found what scientists look for least -- a bit of evidence "contrary to . . . neurobiology dogma." Indeed, the news overturned a principle enshrined in generations of textbooks: that "no new neurons are ever formed after infancy."

But Nottebohm had found, incontrovertibly, in the forebrains of both male and female canaries, "a massive birth and death of neurons" occurring "well after sexual maturity." Exacting tests revealed that old neurons were constantly dying off and new ones constantly taking their place. Were that not astonishing enough, that unheard-of process was happening right in an area of the brain that in "birds and other vertebrates controls complex learned behavior." And the turnover was happening on a massive scale. Indeed, Nottebohm admitted, it was the volume of the replacement that he found "the most shocking."

But he saw no possibility of error. These were unmistakably brand-new, genuine neurons. He suspects that sex hormones may play some role in the process and that the same thing may happen in humans. That's because "from all we know of nervous systems, we get the impression that principles of function are widespread across taxonomic phyla." That is, basic processes that occur in the brains of some animals generally show up in others as well.

We won't know any time soon if it's going on in our heads too; finding the new neurons meant killing the canaries. On that same lab table also perished an established certainty of neurobiology. Comments Nottebohm, "It is so contrary to anything we anticipated that we are not yet prepared to sound intelligent when we talk about it."

When a leading neuroscientist finds himself confounded by bird brains, what can lay observers make of the state of our knowledge of human brains?

We can't even draw consistent conclusions from theory. Western women apparently excel in such left-hemisphere properties as verbal fluency, but not in such left-hemisphere properties as linear reasoning. Men seem to lead in such right-hemisphere techniques as spatial analysis, but not in such right-hemisphere qualities as intuition. Studies have shown a high "tactile sensitivity" (or delicate touch) in women. Researchers have used this to explain feminine superiority in "clerical tasks" like typing, handicrafts like embroidery, and the delicate but repetitive work of the laboratory assistant. Somehow, though, these same superiorities rarely flower into virtuosity on the violin or prowess at surgery.

In sum, the whole debate to date resembles a sort of scientific Rorshach test -- an array of tantalizing, evocative possibilities still so undefined that they may tell us as much about the interests, values and beliefs of the researchers as they do about the true nature of human gender.