Opus the Poet wrote:One of the reasons primates have visible breasts is because our lousy sense of smell prevents us from determining what females are of a breedable age and we need a visual cue. This causes a secondary effect that the larger the breasts the better breeder the female is. Male secondary sexual characteristics include body and facial hair that typically does not grow out as soon as the human male is ready to breed but develops later. This has the side effect of allowing stupid males time to remove themselves from the gene pool before they are allowed to implant a female, most of the time (Darwin Awards anyone?). This has the secondary side effect of ensuring older men pursue younger women because they (the women) have a strong incentive to reject males of their own age until they develop the secondary sexual characteristics. Faster maturation is selected against in males by the fact that two parents were needed to keep a child alive until capable of surviving without constant attention from the mother, because watching the kid(s) would prohibitively impact on the mother's foraging abilities and both would die without having a third person to bring in more food to share. Because of the Stupid Men's Tricks engaged in by males between puberty and developing sufficient secondary sexual characteristics there would be a very high chance that the father would die before the progeny became old enough to fend without constant supervision from the mother.
Ergo, guys go for women with big boobs and younger women tend to go for older guys and vice versa. Boobs equal sexual maturity and sexual competition in women so young girls with big boobs must be slut-shamed to prevent their competing successfully with unattached older females.
A lot of studies have been done on this sort of thing, with mixed results. The basic lesson learned has been: it's all very, very complicated. Also, a good part of it is related to social situation rather than simple instincts. So there tends to be a whole slew of different factors all at work simultaneously to varying degrees, and different situations will let one particular factor over-ride another.
There are two basic approaches to looking for sex partners, labelled as "restricted" and "unrestricted". Restricted means that the person is looking for a long-term commitment, unrestricted means that they're just looking for a brief encounter. Both can result in the passing on of genes, but each goes for a different strategy of doing so. For men, unrestricted means that he increases the total number of times he can pass on his genes, with the downside being that the genes have less chance of surviving after being passed on. Restricted means that he gets less chances to pass on his genes, but the genes have a better chance of surviving in the population with each individual chance. For women, the trade-off is between a variation of genes in their offspring compared to a better chance of surviving the passing on of genes.
A man's or woman's attitude towards the attractiveness of physical features varies depending on whether they are taking a restricted or an unrestricted approach. It seems that for men, both approaches tend to find larger breasts more appealing than smaller, though unrestricted men tend to get more of a thrill out of larger ones than do restricted men. There seems to be something of a limit, though (around E-cup size, if I remember right), where the attractiveness declines for both groups of men.
This preference for size seems to be fairly universal (in that it seems to be the most common preference in different societies, not that every relevant individual in the society necessarily shares the same preference). However, it has a sort of extra factor involved. For more industrially-developed societies where agriculture works on a surplus system, the preference is for large-breasted women with thin waists and wide hips. For less industrialised or unindustrialised societies where agriculture is more of a subsistence level, the preference is for women who are large overall; the appeal of large breasts is part of a more general appeal for women with extra fat over their entire bodies, so it's not really a preference for large breasts per se.
Why so many human women have permanently large breasts (as opposed to pretty much every other animal where they're only all that noticeable during and shortly after pregnancy) is probably
a result of a runaway postive-feedback loop of both intersexual and intrasexual selection. Women with larger breasts were more appealing, and so they bred more often; more genes for large breasts were passed down and more women had larger breasts. This meant that women in later generations needed even larger
ones to stand out from the crowd. As for why
large breasts are appealing, that's still being strongly debated. My favourite reasoning is probably Desmond Morris' claim that men like large breasts because they look more like a woman's butt (I think that this argument is wrong, but I find it rather hilarious). In general it's recognised that larger breasts tend to be linked to greater fertility, so that's likely at least part of the reasoning. But tests have also shown that larger breasts in women (up to a point, again) tend to make men think that the woman is more capable in all sorts of aspects of her life and job, not just in the matter of making babies.
Oh, and on the matter of men with facial hair: women with an unrestricted approach to selecting mates and women who are currently in the non-fertile part of their ovulation cycle seem to prefer men with more extreme displays of sexual dimorphism, which includes having more facial hair. Women who are fertile or who are taking a restricted approach (thus who are looking for a mate who will also be a father and provider for his children) tend to look for less masculine physical traits.