Among primates, only humans have them, including the idea that they act as baby handlebars or energy reserves but claims about sexual selection attract the most attention and seem to have been exhaustively tested. The result?
Men’s preferences seem influenced by number of factors, including: culture (says one study, “Brazilians preferred larger breasts and buttocks than Czechs”), socio-economic status (larger breasts for lower status men), sexual habits, (larger for men in short-term, non-committal relationships), body image, (larger for men who rated themselves more attractive), sexist attitudes (larger for men hostile toward women) and even hunger (hungry British men liked bigger breasts than fed participants). And some experiments focused on other qualities than size, such as how perky or symmetrical breasts are.Bridget Alex, “Scientists Still Stumped By The Evolution of Human Breasts” at Discover Magazine
It wouldn’t be easy to derive a principle of biology from all these studies except that breasts matter. (And we don’t know why.) It’s reminiscent of the many theories that have evolved to explain the fact that humans are fully bipedal. See, for example, a number of theories mentioned in Bipedalism: Regulatory area missing in humans?:
There is a “uniquely human” way of walking upright and there’s no shortage of theses as to why: carrying infants or scarce
resources ,and saving energy strut the stage. Or it is due to climate change or rough terrain? Don’t assume a “chimpanzee starting point,” counsels one expert. News, Uncommon Descent
How about this: Breasts and bipedalism both evolved to create a market for theories.
Okay, okay, but how much worse is that than some of the other theories?
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See also: Human evolution: The war of trivial explanations