Well, there’s a clickbaity title if ever I wrote one.
Nevertheless, human breasts are strange. Sure, all females of the class mammalia are equipped with mammary glands for producing milk, but humans alone posses permanent, non-functional breasts.
Yes, non-functional: the breast tissue that develops during puberty and that you see on women all around you is primarily fat. Fat does not produce milk. Milk ducts produce milk. They are totally different things.
Like all other mammals, the milk-producing parts of the breasts only activate–make milk–immediately after a baby is born. At any other time, milk production is a useless waste of calories. And when mothers begin to lactate, breasts noticeably increase in size due to the sudden production of milk.
A woman’s normal breast size actually tells you nothing at all about her ability to make milk–if anything, the correlation is the opposite, with obese women (with correspondingly large breasts) having a more difficult time nursing and producing milk:
A number of factors associated with low milk supply have been identified, such as nipple pain, ineffective nursing, hormonal disorders, breast surgery, certain medications, and maternal obesity. … Research into breast size and milk production shows that milk supply is not dependent on breast size, but rather on the amount of epithelial tissue contained in a breast that is capable of making milk …
However, in addition to baby attachment issues, accumulating evidence shows that a major factor preventing overweight and obese mothers to breastfeed is the inability of their breast epithelial cells to start producing copious amounts of milk after birth. This is often referred to as unsuccessful initiation of lactation. …
a recent study took advantage of breast epithelial cells non-invasively isolated from human milk. In these cells, certain genes are turned on, which enable the cells to gradually make milk as the breast matures during pregnancy, and then deliver it to the baby during breastfeeding.
The study reported a negative association between maternal BMI (body mass index), and the function of a gene that represents the milk-producing cells. This suggested that the breast epithelial tissue is not as mature and ready to make copious amounts of milk in mothers with higher BMI. Most likely, the large breasts of overweight or obese mothers contain more fat cells than milk-making cells, which can explain the low milk supply of many of these mothers.
Therefore, breast size does not necessarily translate to more milk-producing cells or higher ability to make milk.
More fat=less room for milk production.
Interestingly, average cup size varies by country. Of course the data may not be 100% accurate, and the lumping of everyone together at the national level obscures many smaller groups, like Siberians, but it otherwise still indicates some general trends that we can probably trust.
If breasts don’t actually make milk, then why on Earth do we have them? Why are women cursed with lumpy fat blobs hanging off their chests that have to be carefully smushed into specialized clothing just so we can run without them flopping around painfully?
And for that matter, why do we think they look nice?
One reasonable theory holds that breasts are really just front-butts. Our apish ancestors, like modern chimpanzees, most likely not copulate ad libitum like we do, but only when females were fertile. Female fertility among our chimpish relatives is signaled via a significant swelling and reddening of their rear ends, a clear signal in a species that wears no clothes and often walks on four limbs.
When humans began walking consistently on two legs, wearing clothes, and looking at each other’s faces, this obvious signal of female fertility was lost, but not our desire to look at rear-ends. So we simply transferred this desire to women’s fronts and selectively had more children with the women who piqued our interests by having more butt-shaped cleavage.
In support of this theory, many women go to fair lengths to increase the resemblance between their ample bosoms and an impressive behind; against this theory is the fact that no other bottom-obsessed species has accidentally evolved a front-butt.
I realized yesterday that there is an even simpler potential explanation: humans are just smart enough to be stupid.
Most of us know that breasts produce milk. Few of us really understand the mechanism of how they produce milk. I had to explain that fat lumps don’t produce milk at the beginning of this post because so few people actually understand this. Far more people think “Big breasts=lots of milk” than think “big breasts=lactation problems.” Humans have probably just been accidentally selecting for big breasts for millennia while trying to select for milk production.
Our breast obsession is cargo-cult lactation.