Chapter 1
Chapter 2
Chapter 3
Chapter 4
Chapter 5
Chapter 6
Chapter 7
Chapter 8
Chapter 9
Chapter 10
Chapter 11
Chapter 12
Chapter 13
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.. sex is a biologically and socially costly business. We spend a great deal of time doing things that relate to it, or to parenthood, the biological consequence and rationale for sexual behaviour. But the payoff is less for the individual than for his/her species. So it’s a different kind of demand from the business of daily survival, though the process of reproduction may have vast influence on survival itself. This is the reason that the brain uses many of the mechanisms to regulate sex that we might otherwise think of in connection with more obvious physiological demands such as getting enough food and water….
Now consider the action of testosterone. Increased secretion acts on the periphery to facilitate sexual behaviour – by, for example, increasing the formation of sperm, causing the penis and other parts of the reproductive tract to enlarge, and encouraging growth (in man) of the b
eard and other signs of masculinity (to increase his sexual
attractiveness). It also sends a message to the brain, generating
a ‘demand’ (some would call it a ‘drive’) for
sex. This is as essential in the context of successful
reproduction, sexual selection and procreation, as
‘wanting’ salt is in the context of the homeostatic control
of body fluid composition........ Now we begin to understand why
the limbic system is so closely involved in sex, and why some of the
things we learn about the role of the limbic system in other
‘demand’ states might tell us interesting things about
sex……Sexuality in human males is also responsive to testosterone. Without the benefit of knowing any endocrinology (or even that hormones existed) our forefathers discovered that reliable custodians of harems had to be castrated before puberty. Choir masters keen to prolong the pure singing voice of a boy also knew that castration was effective. Castration after puberty (or, in more modern times, treatments that reduce the effects of testosterone) has less effect, though there is a gradual partial loss of both sexual interest and, perhaps more prominently, the ability to achieve penile erection. This shows that sexual motivation in men is sensitive to testosterone, but also that peripheral effects on the genitals are also part of its action. In some species (such as some monkeys, but not man) the glans penis (the tip) grows arrays of little barbs, the penile spines, when testosterone levels rise….
It may come as a surprise to learn that turning sexual behaviour and reproduction off is as important biologically as turning it on. This is because reproduction, as we have already mentioned, is so costly. Costly for the male, who has to fight off rivals, and may even have no time to eat. Costly for females, who have to nurture their young both through pregnancy and lactation, putting enormous strains on their bodies and their energy supply……
So is there a ‘sexual brain’? That is, a defined part of the brain given over to sexual behaviour and, one assumes, the associated hormones and other features of reproduction? And does all the experimental evidence, gained mostly from years of patient research on rats (and a few other species) actually apply to humans? ……. We still can’t point to a bit of the brain, or a bunch of nerve cells in the limbic system, and say (truthfully) that this is the sexual brain (as distinct from other categories). But there must be one: perhaps the secret lies in a level of analysis which has hardly begun for sex: the chemical content of certain nerve cells……
Let’s pay a visit to cloud nine, on which sits the Director-General. He/she/it has decided that animals will have a limited life span and that sex is the way that all the mammals will procreate, and issues the corresponding order. Down on cloud twenty-one, the Chief of the design team scratches his/her/its
celestial head. After a moment’s inspired
(divinely-inspired, perhaps?) thought, he (etc) has the answer.
He’ll make sex very nice (so we’ll all want to do it) and
he’ll so arrange how we do it to increase the chances of each
gender being reproductively successful. We’ll adopt the
best tactics, but be unaware of the overall strategy. Natural
selection will operate on the outcome of the strategy: if the tactics
are no good, then there won’t be any individuals left to continue
using them. The tactics are what the brain does, in the service
of the overall, sublime, strategy. If, therefore, we want to
understand how the brain controls sexuality, we need to talk
tactics. Darwin said it all, when he put forward his ideas on
natural and sexual selection, though he wasn’t so very interested
in the role of the brain….. It’s a biological fact that aggression and sex go together……. So testosterone acts on the brain in at least two ways: to increase sexual motivation, but also to supply a tactic whereby a male can improve his access to a sexually-attractive female by fighting for her………
…. human behaviour shows rather beautifully the respective roles of the ‘cognitive’ and ‘limbic’ parts of the brain in sexual selection. The woman’s limbic brain, honed by millions of years of successful evolution, is making sure she mates with the best male around. Best in the sense that he has genes that maximise his survival, and hence that of future children, and qualities that ensure he provides for those young. In the forests of yesteryear, this might have been the man with the broadest shoulders, the strongest arm, the most prowess at hunting. Fast forward several millennia. The strategy remains the same, but the tactics have altered. The qualities that served so well in the forest do little in the more recent world of high finance. Yet there are still men who offer better genes or better provision, only the attributes have altered. So she shifts her selection criteria. Her neocortex informs her about the subtleties of modern ‘fitness’ (it’s the only part of the brain that can) but her limbic system plays the same old tune….
Why do soldiers rape? The curious fact is that even well-brought up young men, from societies in which rape is not tolerated and practically unthinkable (even though it may occur), and who would never contemplate rape in their home towns, can and do commit rape in times of war. Is there anything we can say about the role of the brain in rape?.........
Scientific and literary quotes.