Extent and limits of sexual dimorphism of the brain

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 Everybody will agree that there are biological differences between boys and girls and between men and women.  We all think of such things as primary (sexual organs) and secondary sexual physiognomic traits (Adam's apples, voice tones, body hair, etc.).  Unfortunately,  many persons interested in sex differences are unconcerned with biological sex differences which are not immediately palpable or which cannot be seen directly.  In mammalian species (the rat has been studied most in this respect)  including humans,  there are obvious and subtle brain differences between the sexes.  The obvious brain differences can be deduced as follows:  if the sexual and reproductive organs are very different in men and women,  and to the extent that these organs operate on rigidly biologically determined species-specific time-tables (the menstrual cycle, the gestational cycle, etc.), and if their operation is minimally complex,  then the neurons of the brain which control them must somehow be different in men and women.    This has been shown by thousands of empirical investigations to be so. Consider this example.  The hypothalamus is a small phylogenetically ancient (primitive) structure (it is found in fish brains)  situated at the center of the brain.  It is a group of closely packed neurons, tightly interconnected by synapses (physiologically active connections).  It is strongly linked to a gland situated just beneath it in the brain called the pituitary gland (also known as the hypophysis).  The hypothalamus controls the pituitary gland.  The pituitary gland controls other glands that are sex-specific (ovaries, testicles) and secretes factors which regulate sex-specific hormones (oxytocin, prolactin, progesterone, estrogen, testosterone).   You can imagine that there has to be a difference between the hypothalamus of a man and that of a woman.   There are other examples of brain systems that are sex-dimorphic for very obvious reasons.   Though the clitoris does undergo a bit of tumescence during sexual excitation,  the penis undergoes much  more.  Penile tumescence,  like every behavior,  is under neural control.  So it would be only natural, and it should be expected, that scientists should find that the part of the brain controlling penile tumescence ought to be bigger in men than in women.  The spinal nucleus of the bulbocavernosus (SNB) is one of the control centers in question.  It is a small nucleus (aggregate) of neurons situated in the spinal cord which controls penile tumescence. There are also select muscles which are more markedly developed in the adult man than adult woman. The perineal muscle, also involved in penile erection,  is an example of this and it is controlled by a spinal nucleus called the dorsolateral nucleus (DLN).  These two spinal cord nuclei are sexually dimorphic in the rat and human.  These nuclei and the perineal muscles they innervate are present in males but reduced or absent in females. The sex difference in motoneuron number in these nuclei is due to an androgen-regulated motoneuron (neurons directly feeding muscles) death. Developing females treated with the androgen testosterone propionate (TP) have a fully masculine number of SNB and DLN motoneurons and retain the perineal muscles they would normally have lost. Paradoxically, females treated prenatally with the androgen dihydrotestosterone propionate (DHTP) also retain the perineal musculature but as adults lack the SNB motoneurons which would normally innervate them. The SNB target muscles retained by DHTP females are anomalously innervated by motoneurons in the DLN. Counts of motoneurons and degenerating cells in the developing SNB of DHTP-treated females showed that their feminine number is the result of a failure of DHTP to prevent the death of SNB motoneurons. Furthermore, the peak number of SNB motoneurons was below that of normal females, suggesting that DHTP treatment may also have inhibited motoneuronal migration. However, DHTP treatment fully masculinized both motoneuron number and degenerating cell counts in the DLN of these females, and it is this masculinized DLN that gives rise to the anomalous projection. Taken together, these results suggest that the effects of different androgens during development are specific and complex, involving the regulation of motoneuron death, migration, and specification of peripheral projections.   Very recent findings add a fascinating twist to this story.  As surprising as it may seem to many developmentalists,   sexual experience may play a role in the development of sex differences in such brain nuclei.  Investigators looked at the brains of sexually active and inactive male rats. They found motor neurons -- the cells in a hormone-sensitive part of the brain -- were smaller in the lover than in the virgin rats (Breedlove, 1997).  A leading researcher in this field,  Breedlove,  proposes that "Somehow the extensive sexual experience affected the morphology of these neurons."   It is possible, he says, that differences in sexual behavior cause,   rather than are caused by, differences in brain structure.  Another recent study also investigated the effects of sexual behaviorial manipulation on brain plasticity in adult male rats.   Adult male Sprague-Dawley rats were divided into four groups: control male; gonadectomized (Gdx) male; sexually active male; and sexually nonactive male. Female animals were used as an additional control group. At the end of a 12-week experimental period, the animals were again tested for male sexual behavior and tested for sexual motivation. Sexual behavior manipulations over the 12-week period resulted in significant differences in mount latency, mount frequency, intromission latency, intromission frequency, ejaculation latency, and the postejaculation interval.   In the motivation test, significant differences in the number of approaches, contacts, and crossings of an electrified grid separating the test animal from a receptive female were also observed.  The sexually dimorphic nucleus of the preoptic area (SDN-POA) volume in sexually nonactive males was significantly smaller than in control males or sexually active males. Anteroventral periventricular nucleus (AVPV) volumes in the male groups were not significantly altered by sexual behavioral manipulations and did not differ from female volumes (Prince et al, 1998).

Men's brains weigh, on the average, 200 grams more than women's brains:  about 1500 grams for men and 1300 grams for women.  But what does this mean ?   Large men's brains weigh more than small men's brains.   In fact,  the body-size difference between men and women is believed sufficient,  by most commentators, to explain the difference in brain weight.  Besides,  brain weight does not correlate well at all with intelligence.  Einstein had a rather small brain,  but he wasn't linebacker format either, was he ?  Whales have huge brains,  much larger than humans,  but they are not as smart.  If a well trained neuroanatomist (not specifically trained in the anatomy of sex differences though) is handed several normal brains and asked whether they were extracted from the skulls of a) a large woman, b) an ordinary woman, c) an ordinary man, d) a small man,  he or she will have absolutely no idea,  no matter how many hours he or she gazes at it and no matter how many slices he or she cuts.   In short,  men's and women's brains are remarkably similar at the macroscopic level (at the level of gross anatomy).  

Nevertheless,  there may be a subtle   difference in overall brain anatomy between normal men and women.  The neuroendocrinologist Helmuth Nyborg recently published a book chapter in which he carefully analyzed the evidence.  He concluded, apparently justifiably as far as I can judge, that there remains a difference in brain weight  between the sexes, after adjustment for body weight, of about 100 grams.   Others have independently come to the exact same conclusion. Furthermore,  pubescent boys,  who weigh less than their female counterparts have 20% heavier brains.  Finally,  within a given sex,  body weight barely correlates with brain volume.  In fact,  a consensus is building in neuroscience to the effect that the human male has 100 milligrams of extra brain,  but it still not clear whether he has more neurons than the human female:   Sandra Witelson found that neuronal packing is significantly tighter in women in one specific area of cortex (the planum temporale).   However, scandinavian researchers have recently published a paper reporting that they estimated normal men to possess about 19 million more neurons than women.   Since there are about a thousand billion neurons in the average brain,   this difference is roughly commensurable with Nyborg’s proposal of a 100 gram brain weight difference between men and women.   However,  the reader is advised to withhold judgement on this issue until exhaustive research has been published.

Besides,  more does not always mean better.  Excessive brain tissue can be caused by an error of genetic programs controlling brain development in utero  (before birth).  Some of these conditions are so severe that they are called macrocrania or macrocephaly (an excessively large head) and are typically associated with mental deficiency. Sotos syndrome (cerebral gigantism) is one of the better known of these conditions.  I am not suggesting that the male sex is more at risk for macrocrania.  However,  the male sex could indeed be subject to a subtle delay (and even a disadvantage) in prenatal pruning (natural cell death) in the brain.   A neuroanatomist named Sandra Witelson has eloquently argued that this may indeed be the case,  and I review some of her arguments to this effect a few sections down.