Scientific American Editors - His Brain, Her Brain

His Brain, Her Brain

From the Editors of Scientific American
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Letters to the Editor
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His and Hers

Boy or girl? Even before a person is born, thats the first thing everyone wants to knowunderscoring just how much value human societies of all types place on gender.

The great divide between male and female has inspired poets, intrigued biologistsand spawned an entire industry of self-help books. It has also given rise to a few misconceptionsalong the way, such as the notion that smaller brains make the so-calledfairer sex inherently less intelligentor at least bad at math.

In this eBook, we take a closer look at genderand the brain. In these pages, you will learn about a variety of male-female differencesand also some surprising similarities.[Editor's Note: this eBook was originally published as a special edition of Scientific American Mind.]

For instance, studies of baby boys and girlssuch as the ones described in Big Answersfrom Little People, by David Dobbsfind few differences in theircognitive skills. But there is no denying that boys love trucks, whereas girls prefer dollsand the same is true of male and female monkeys. Some differences between boys andgirls are evident even on the first day of a babys life.

Most of these discrepancies start out small but get amplified by our gender-obsessedculture. As neuroscientist Lise Eliot explains in The Truth about Boys and Girls, tea parties and wrestling matches leave their stamp on growing brains. Thegap that separates boys and girls would be less noticeable if parents encouraged activitiessuch as reading for boys and video games for girls.

By the time they reach adulthood, males and females not only have nonidenticalbrain architectures but also divergent ways of speaking, parenting and responding toboth tragedy and comedy. The Humor Gap by Christie Nicholson and Different Shades of Blue by Erica Westly explorethese divides. She wants someone who can make her laugh. He wants someone who willlaugh at his jokes. And when shes depressed, she gets sad. He gets mad.

But men and women arent from different planets. Few sex disparities are as hardwiredas popular accounts make them out to be. A better understanding of the realand imagineddifferences between his brain and her brain can help us overcome culturalbiases, improve communication and strengthen relationships.

Dawn Stover
Issue Editor

His Brain, Her Brain

by Larry Cahill

On a gray day in mid-Januaryof 2005, Lawrence Summers,then president of HarvardUniversity, suggested that innatedifferences between the male andfemale brain might be one factor underlyingthe relative scarcity of women inscience fields. His remarks reignited adebate that has been smoldering for acentury, ever since some scientists sizingup the brains of both sexes beganusing their main findingthat femalebrains tend to be smallerto bolsterthe view that women are intellectuallyinferior to men.

To date, no one has uncovered anyevidence that anatomical disparitiesmight render women incapable ofachieving academic distinction inmath, physics or engineering. And the brains of men andwomen have been shown to be quiteclearly similar in many ways. Nevertheless,over the past decade or so investigatorshave documented an astonishingarray of structural, chemical and functional variations in the brains of malesand females.

These inequities are not just interestingidiosyncrasies that might explainwhy more men than women enjoy theThree Stooges. They raise the possibilitythat we might need to develop sex-specifictreatments for a host of conditions,including depression, addiction,schizophrenia and post-traumaticstress disorder (PTSD). Furthermore,the differences imply that researchersexploring the structure and function ofthe brain must take into account the sexof their subjects when analyzing theirdataand include both women andmen in future studies or risk obtainingmisleading results.

Sculpting the Brain

Not so long ago neuroscientistsbelieved that sex differences in thebrain were limited mainly to those regionsresponsible for mating behavior.In a 1966 Scientific American articleentitled Sex Differences in theBrain, Seymour Levine of StanfordUniversity described how sex hormoneshelp to direct divergent reproductivebehaviors in ratswith malesengaging in mounting and femalesarching their backs and raising theirrumps to attract suitors. Levine mentionedonly one brain region in his review:the hypothalamus, a smallstructure at the base of the brain thatis involved in regulating hormone productionand controlling basic behaviorssuch as eating, drinking and sex.A generation of neuroscientists cameto maturity believing that sex differencesin the brain referred primarilyto mating behaviors, sex hormonesand the hypothalamus.

That view, however, has now beenknocked aside by a surge of findingsthat highlight the influence of sex onmany areas of cognition and behavior,including memory, emotion, vision,hearing, the processing of faces andthe brains response to stress hormones.This progress has been acceleratedin the past decade by the growinguse of sophisticated noninvasiveimaging techniques such as positron emissiontomography (PET) and functionalmagnetic resonance imaging(fMRI), which can peer into the brainsof living subjects.

These imaging experiments revealthat anatomical variations occur in anassortment of regions throughout thebrain. Jill M. Goldstein of HarvardMedical School and her colleagues,for example, used MRI to measure thesizes of many cortical and subcorticalareas. Among other things, these investigatorsfound that parts of thefrontal cortex, the seat of many highercognitive functions, are bulkier inwomen than in men, as are parts ofthe limbic cortex, which is involved inemotional responses. In men, on theother hand, parts of the parietal cortex,which is involved in space perception,are bigger than in women, as isthe amygdala, an almond-shapedstructure that responds to emotionallyarousing informationto anythingthat gets the heart pumping andthe adrenaline flowing. These size differences,as well as others mentionedthroughout the article, are relative:they refer to the overall volume of thestructure relative to the overall volumeof the brain.