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The world learned this week that the average male penis size is 5.16 inches when erect. While we were talking anatomical numbers, we wondered: What’s the average female breast size? And has the figure gotten bigger over time? (Pun intended.)

In her highly acclaimed 2012 book Breasts: A Natural and Unnatural History, journalist Florence Williams reported that average breast size has grown from 34B to 36C over the course of 15 years, according to bra industry data. (Bras are the best metric we have for measuring boobs, since doctors don’t measure breast size during regular checkups.)

Meanwhile, in 2013, the lingerie retailer Intimacy released a survey claiming to reveal that the average breast size in America has ballooned from 34B to 34DD over the past three decades. The finding has been repeated over and over again in media coverage—but all it really reveals is that women are buying bigger bras.

Whether or not boobs themselves are getting bigger, American women’s average bra size does appear to be growing. Here are the theories why.

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Americans are getting fatter.

More than one-third of adults in this country—a whopping 78 million people—are obese. And given that breasts are made up largely of fatty tissue, it stands to reason that bra size would expand along with waistlines. According to the CDC, in the 1960s, the mean weight for a woman was 140 pounds; in the 1970s, it was 144 pounds; in the 1980s, it was 154 pounds; and by 2010, it had hit 166 pounds. Women aren't necessarily growing bigger boobs, they're just growing more fat.

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Bra sizes have gotten more accurate.

For decades, women with B and C cups were stuffing themselves into A-cup bras, thanks largely to misguided bra fittings. Then came (no joke) the Oprah effect: In 2005, the daytime talk deity dedicated a show to helping women discover their true bra size, and demand for professional bra fittings surged. Suddenly, women who thought they were a B found out they were really a DD. Since then, companies have expanded their sizes, offering options all the way up to the letter K.

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"[Two decades ago] the American market carried less than 20 sizes, so women with bigger breasts squeezed into bras that were two or more cup sizes too small," said a rep for Intimacy when the company's survey first made headlines. "Therefore, the idea that breast size is increasing is perhaps slightly inflated due to women actually purchasing larger (and more accurate) bras for themselves."

That said, bra companies are also inflating cup sizes.

Just as some clothing retailers have shrunk what used to be size a 12 into a size 8—popularly known as "vanity sizing—bra manufacturers have turned a B into a C, or a D into a DD, according to The New York Times. There is no industry-wide standard for what an A or a B or a DD cup is, and sizes vary from company to company, the paper reported.

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Curious to learn more about the history of the bra? Check out our animated short:

Plastic surgery. 

You knew this was coming. Over the past decade, boob jobs have risen to become one of the most popular cosmetic surgeries in this country—in 2014 alone, 286,254 women got breast implants. Fake boobs influence bra size as much as real ones.

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Women may actually be growing bigger breasts—thanks to pollutants.

That's right. Women's breasts may be getting bigger due to environmental factors such as industrial contaminants. Williams, the reporter who wrote the book on breasts, makes the case that boobs' fatty tissue absorbs pollutants "like a pair of soft sponges." Not only is our environment leading to higher rates of breast cancer, she argues—it's also causing women to grow bigger breasts, and to do so at an earlier age.

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It works like this: A category of pollutants known as persistent organic pollutants (POPs) mimics estrogen and can thus alter a woman's hormonal balance, leading to bigger breasts and serious illnesses. "Breasts are filled with estrogen receptors, and lots of food and household products contain either estrogen or estrogen-mimicking compounds, and these cue breast tissue to grow," Williams told Fusion.

Not only that, as she reports in her book, breast milk now contains toxins—including chemical flame retardants—thanks to our contaminated environment.

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So what's the takeaway?

The jury is still out on whether breasts are really getting bigger—and if they are, the environment and obesity appear to be the culprits. But remember, just because women are buying larger bra sizes doesn't mean they're evolving to have bigger breasts. A quick tour through historical paintings shows that at least some women have always been well endowed.

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Taryn Hillin is Fusion's love and sex writer, with a large focus on the science of relationships. She also loves dogs, Bourbon barrel-aged beers and popcorn — not necessarily in that order.