"This variability is a significant factor upon which many of the hypotheses founder. If you’re going to argue that breast size has been selected for, you either have to show that small-breasted women have a reduced fitness, or that there are trade-offs with other factors that can favor a variable distribution. You also have to do the hard work of actually gathering observational evidence and assessing correlations — you don’t get to do experimental work on humans — and another of the failings of many of these hypotheses is that the evidence just isn’t there.
Another problem many of these hypotheses have is that they fail to address another dimension: why does your hypothetical advantage only apply to women? You have to explain why your explanation applies to only half the human population...
The sexual maturity hypothesis
This is the idea that breasts are basically a visible meter to allow one to judge the sexual maturity of the female — you can easily assess whether a female is prepubertal, and therefore infertile, and the breast shape changes also allow one to see at a glance when a female is old and post reproductive.
Notice that once again we’re in a male-centered domain: men are judging, women are being passively judged. No one ever seems to consider that it is to the woman’s selective advantage to exercise control over her reproduction, so why are we assuming it is evolutionarily favored for her to advertise? We can hypothesize about women evolving cryptic ovulation to hide their fertility, but at the same time argue that males were selecting females for visible fertility signals...
So the first cause for breasts: pure chance. An intelligent primate species could just as well have evolved that had fatty deposits on male chests, while the females grew beards. Evolution doesn’t care."
I like this partially because it's such a great example of how a scientist thinks through a question. Starting with some basic principles that they have observed about the phenomenon - in this case, sex-specificity and high variability - and then looking for ways to explain those things and then Looking For Ways to Test Those Explanations.
I feel like that last part is what is missing a lot of the time. It's why scientists are always a little frustrated with the rest of society.
Related: Obstetric dilemma https://medium.com/science-and-innovation/of-evolution-culture-and-the-obstetrical-dilemma-a3b7ce47e207
FB: "I’ll say it again: we’re ignoring the variability. Are women with small breasts bad at advertising their sexual maturity? Then there’s the flip side: there’s variability in the male response. Creepy as it is, there are men who are attracted to small, childlike bodies, where small breasts are an indicator of youth. What is being signaled? How is it being received? We are in a world of mixed messages here, which could, in part, be one of the reasons there is so much variability…but it also means we lack a consistent context to explain the origin of the phenomenon."
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