In BLONDES I mention that, for many women, the left breast is slightly — about 4 percent — larger than the right. The factoid was buried in a discussion about sexual selection, the upshot of which is that breasts are a factor even in cultures that don’t eroticize the chest any more than the face. Breast symmetry is one cue of developmental stability, and large and symmetrical breasts suggest “good genes.” That said, yes, it’s normal to have one breast that is slightly larger than the other. The larger your breasts, the more obvious the asymmetry. For about 65 percent of us, the more voluminous one is the left.

But why?

Breast asymmetry is associated with rapid growth spurts prior to and during puberty. It’s normal for one breast to respond more to surging estrogen levels. Hormonal disruptions and other development problems accentuate the asymmetry. But why the left? No one knows for sure, but there are interesting theories:

* The immune system is known to be functionally asymmetrical, and immune hypersensitivity is higher on the left side of the body. It’s possible that enhanced immunosensitivity has an effect on tissue-plumping growth hormones such as estrogen. The immune system may send to the brain information that is asymmetrically expressed. Immune system asymmetry may also explain why cancers occur 5-10 percent more often in the left breast and inflammation associated with herpes zoster affects the left side more than the right.

* Nursing infants may make the left breast even larger. Although studies have not found a link between a larger left breast and handedness, many women report that they prefer having their dominant right hand free while breastfeeding, which means infants more often nurse on the left breast. Favoring the left breast makes it larger.

Personally, I prefer the explanation my friend gave her curious eight-year-old daughter when she saw that Mom’s left boob was bigger: It’s because I have a really big heart under it.