In Breasts: A Natural and Unnatural History, Florence Williams expounds on the good things mammary glands have done for us—and their new role as purveyor to our infants of environmental toxins.

How have people responded to your writing a book about breasts?

It was a little bit awkward when I would tell the elderly relatives about it. I would get these funny looks, and I think my husband had similar responses when he would tell his colleagues. But when I described the book more and told them it was really about health and the environment, then everyone was really interested and excited about it.

We live in a society that fetishizes breasts, and yet as environmental pollutants are absorbed into breast milk, we’re destroying their function. Is there a paradox there?

I think there is definitely a disconnect between how we live our lives and what that lifestyle does to our bodies. There is a lot of compelling evidence to suggest that the things we eat and the substances in our lives affect our bodies on a cellular level. Part of my hope with this book is to help raise awareness about environmental health.

You describe how women scientists have brought a new perspective to our understanding of why breasts evolved—to nourish babies, not to attract men.

Absolutely. What I love about the evolution-of-breasts debate is that it really reveals everyone’s biases. I think Desmond Morris’s theories [that breasts developed to attract male partners] really reflect a certain time in modern 20th-century Western life. It is fun to poke holes in that and reflect on it. If we buy into the idea that the purpose of breasts is to serve men, then that does a disservice to the message we try to send young girls about their body types, and to mothers about the importance of lactation.

Has writing this book changed how you talk to your daughters on this subject?

Yes, absolutely. I think it is very important to talk to our daughters about social pressures and expectations, and how those may not be rooted in reality. One of the things that struck me, writing this book, was the power of the Internet in shaping young people’s perspective about body types. Most young people are seeing so many more images of augmented breasts or fake breasts than we ever did. There is so much porn on the Internet and young people are looking at it. Most of those breasts are not natural breasts.

You present evidence that girls are reaching puberty earlier, which increases their risk of getting breast cancer. Highly athletic girls tend to have delayed onset of menstruation. Does that protect them against breast cancer?

The athleticism does seem protective. This is not only true for adolescents, but across the lifetime of women. The more athletic you are, the lower your chance of breast cancer. It still benefits you if you start exercising later in life.