QUESTION: Biologically speaking, why is there such a variance in breast sizse? For example, shorter height has it’s evolutionary advantage (longer life span due to reduced strain on the heart). I don’t seem to understand the advantage of smaller or larger breasts. All heights provide certain advantages. So if there is no particular advantage among the various sizes… why hasn’t evolution standardized the size?

ANSWER: Breast size is under a type of selection known as balancing selection. This means that there are a range of sizes which are all equally optimal, while extremes are discouraged.

Looking at the population, we typically don’t see many cases of insanely large or small breasts. While a few individuals have very large breasts, there’s no evolutionary advantage, and several disadvantages (back problems, muscle strain, sexual harassment lawsuits). Small breasts don’t have similar problems, which is why our balancing selection curve covers the area from small to fairly large as “optimal”.

Breast size doesn’t impact milk production, so there’s no advantage to larger breasts in terms of providing nutrition to the child. In addition, while studies are contradicting, there’s a lack of clear evidence suggesting that larger breasts lead to an increased risk of cancer. (Studies currently suggest that, while a larger breast doesn’t necessarily increase cancer risk, it may make detecting tumor lumps more difficult.)

So overall, there’s not a ton of selection influence on breast size, aside from selecting away from the extremes. Some males will prefer smaller-breasted women, while others prefer larger-breasted women. Because there’s no selection working to push towards one specific size, we maintain a fairly diverse range of sizes.

Short Summary: There’s a range of breast sizes because no standard size offers an evolutionary advantage over another. Humans maintain a diverse range of breast sizes because there’s no negative selection pressure pushing us away from large (or small) sizes. https://www.reddit.com/r/explainlikeimfive/comments/4p5j0a/eli5_biologically_speaking_why_is_there_such_a/

p.s. Interestingly, no other primate has permanent breasts. Humans are the only ones with full breasts regardless of whether they are lactating or not. Many theories abound as to why this is the case. https://www.theguardian.com/science/2010/may/14/breast-size-evolution