The average bustline has grown since the 1980s.

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Why are breasts different sizes?

Each year, British lingerie retailer Debenhams surveys its female customers to keep tabs on the kinds of knickers women are wearing. In 2008, for instance, the data deemed the pushup bra as women's most beloved "fashion invention" of all time, but according to the survey results published in 2012, modern-day breasts might not need much additional augmentation [source: UPI]. Among Debenhams' patrons at least, the most popular bra size had ballooned since 2010 from a 34B to a 34DD [source: Adams].

A survey from a single store chain doesn't constitute a bona fide busty pattern, but other unrelated statistics also indicate that Western breasts have become more bountiful in recent years -- and not because of an uptick in cosmetic surgeries. In 1983, Playtex lingerie manufacturers reported their best-selling bra had grown from a 34B to a 36B, and another metric now pegs the average American cup size at a 36C [source: Adams and Redbook]. This population-wide upswing can be attributed to a number of factors, such as women's expanding waistlines, great interest in properly fitting undergarments and a lack of standardized sizing among lingerie brands [source: Holson].

When it comes to individual pairs of breasts, the size-related variables continue. Weight changes, medications, pregnancy and menstruation can all incite fluctuations, but breast size is largely determined by seven genetic markers [source: Erikkson et al]. Bras therefore come in an alphabet of sizes and styles because every woman inherits a unique set of breast-related DNA from her parents' genes. And even though breasts are typically considered identical pairs, size differences exist not only from woman to woman but also, quite often, from breast to breast. During female puberty, estrogen stimulates thelarche, or initial breast development; budding breasts, however, don't always receive identical doses of the hormone, which means one side might grow faster than the other [source: Hirsch]. Early breast asymmetry may even out in adulthood, but at least 25 percent of women have slightly lopsided busts [source: Peterson].

As researchers discover more details about the genes that call the shots on breast sizes, they're also starting to look into whether asymmetry has long-term implications beyond having a harder time shopping for the appropriate bra cup.

Asymmetrical breasts are a normal part of healthy female development.

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Does breast size predict cancer risk?

Rarely, a condition called juvenile hypertrophy can cause one breast to swell much larger than the other, potentially leading to excessive physical and psychological complications [source: Sigurdson et al]. Milder cases of breast asymmetry likely affect the mind far more the than the body, but nevertheless, evening out the bust line is one of the most common elective plastic surgeries for female patients under 18 years old [source: American Society for Aesthetic Plastic Surgery]. In most cases, though, the perceived cosmetic flaw is simply a byproduct of normal breast enlargement brought on by estrogen that stimulates milk duct growth and fat accumulation in the mammary tissues. Moreover, asymmetry is more expected among secondary sex traits, which include breasts and penises, because they undergo accelerated growth stages following the onset of puberty that could promote irregularities -- or differing brassiere cup sizes [source: Scutt, Lancaster and Manning].

But in 2006, a study published in the journal Breast Cancer Research raised a new concern about breast asymmetry. Researchers from the University of Liverpool compared mammograms from women with symmetrical versus asymmetrical breasts and drew a correlation between uneven busts and higher breast cancer risks [source: Boyles]. Based on the data from 252 women who eventually developed breast cancer, the researchers calculated that every 3.3-ounce (100-milliliter) difference in size between breasts increased a woman's cancer odds by 50 percent [source: BioMed Central]. Estrogen has been implicated in playing a significant role in breast cancer development, and the asymmetry of disproportionate breasts could signal a susceptibility to the hormone's mutation capabilities [source: Scutt, Lancaster and Manning].

That said, the isolated relationship between asymmetry and cancer risk likely isn't as absolute as the study data suggest; a woman's age, family history of breast cancer, childbearing status and a host of other medical dynamics also impacts her chances of developing cancer. Likewise, having breasts that don't uniformly fill out a bra doesn't mean that anything is anatomically out of the ordinary. Although idealized images of perfectly symmetrical breasts may lead us to believe otherwise, oftentimes every breast is a little bit different.

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Author's Note: Why are breasts different sizes?

In today's media-saturated world, men and women alike are inundated with idealized body images. For the fellas, the unrealistic expectation centers on a smooth, possibly hairless sculpted chest, rippling muscles and a taut triangular back. For women, well, the attention rarely strays below the belly button. Breast implants are the most popular cosmetic surgery in the United States, and for female patients under 18 who aren't legally allowed to elect for a larger cup size, one of the most common corrective procedures addresses another area of bust line insecurity: evening out an asymmetrical set. What many girls and women might not realize, however, is that having breasts of slightly different sizes is perfectly normal and just another example of how abnormal our concepts of bodily perfection really are.

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Sources

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