Bilateral symmetry is only accurate to a first approximation
January 23, 2011 11:40 AM   Subscribe

A large portion of the women I know (myself included) have breast asymmetry - one breast is larger than the other. I know this is unremarkable and pretty normal; humans aren't actually that symmetrical when you look closely. But I was talking about this with my girlfriend this morning and we noticed that for both of us, it's the left breast that is larger. Is there a species-wide skewing of left vs right breast for which one tends to be bigger? Does it follow handedness? Do men have noticeable asymmetry (presumably scaled smaller)? If it skews left, is there some whacked out evolutionary advantage of having the extra bit of padding over the heart?
posted by rmd1023 to Health & Fitness (23 answers total) 10 users marked this as a favorite
Yes, this is how it goes, bigger left one. Someone explained it as having something to do with the heart being slightly on the left, but no idea if that is true to any degree.
posted by Patbon at 11:45 AM on January 23, 2011

I don't know if this is relevant but yesterday I got my eyebrows threaded and the technician mentioned to me that she noticed that most women have more hair on the left side of their faces.
posted by amethysts at 11:49 AM on January 23, 2011

Right-handed and left boob is distinctly larger, just as a data point.
posted by restless_nomad at 11:58 AM on January 23, 2011

Best answer: Not a huge sample size either, but bigger than two:

"During the study period, 88 patients underwent surgical correction for breast asymmetry. The response rate was 64.8 percent (57 respondents), with a mean age of 28 years. The majority of patients (84.2 percent) were right handed. Breast size difference revealed that the left breast was more frequently the smaller of the two breasts (61.4 percent) [...] Fisher's exact analysis revealed no statistically significant association between hand dominance and breast size difference (p = 0.238)."
posted by Catseye at 11:58 AM on January 23, 2011 [4 favorites]

Your heart is barely off center to the left of your chest cavity, where the curve of the breast begins, so I'd think the whole heart protection hypothesis is a non-starter for that reason alone.

There may or may not be a pattern of more [hair / breastage / whatever] on one side of the body, but the reports of friends and cosmeticians are prone to confirmation bias. A report (behind a pay wall) cited in this Wikipedia article (NSFW: naked breast pictures), says that "It is typical for a woman's breasts to be unequal in size particularly while the breasts are developing. Statistically it is slightly more common for the left breast to be the larger."
posted by maudlin at 12:00 PM on January 23, 2011 [1 favorite]

Observation bias. Catseye's link has a larger data set with the opposite trend, but I wouldn't take that as definitive, either. Someone would have to do a really large data collection to figure out if there were a trend in either direction; just-so stories aren't going to cut it.
posted by Sidhedevil at 12:01 PM on January 23, 2011

I think it has to do with which arm gets more use, but I do know some right-handed women whose right breast are larger so it's not a set rule.
posted by Neekee at 12:14 PM on January 23, 2011

I am normal weight and right handed. My right boob is bigger than left, by about 2 cup sizes. It is not the pectoral muscles making it bigger, just regular boob fat.
posted by fifilaru at 12:19 PM on January 23, 2011

Chiming in to say that my right boob is slightly bigger than my left. When they were still growing, it was much more noticeable, but they've since evened out.

I'm right-handed and normal sized.
posted by easy, lucky, free at 12:35 PM on January 23, 2011

Do men have noticeable asymmetry (presumably scaled smaller)?

Yeah, look to the balls.
posted by hermitosis at 12:35 PM on January 23, 2011

Response by poster: hermitosis: Yeah, I know there's testicular asymmetry. I was wondering if there was also breast asymmetry, although with that much less tissue in the area, it would likely be a smaller difference.
posted by rmd1023 at 12:40 PM on January 23, 2011

My doctor told me my left side was more developed than my right (to a "hmm, that's odd" extent). So, there you go.
posted by griphus at 12:44 PM on January 23, 2011

This is where I tell the internet that, while my boobs are the same size, I can wiggle my left one by flexing. That's right, behold my boob-talent glory.

I've always assumed it's related to my being able to raise my left eyebrow and Elvis-snarl the left side of my lip.

I'm right handed.
posted by phunniemee at 12:49 PM on January 23, 2011 [1 favorite]

As I explained to by husband, we are evolutionarily descended from the great Amazon women. Those women were fantastic hunters with a bow and arrow. Typically right-handed, the smaller right breast enabled better execution of the bow. You small lefties must be descended from a different tribe.

Also, if men's balls weren't lopsided, I reckon they'd walk funny.
posted by amanda at 1:02 PM on January 23, 2011 [3 favorites]

FWIW, also a right-handed, left-boobed person here. And unlike others, mine has gotten more asymmetrical as I've grown, although I think that is a matter of weight gain - it became more noticeably larger than the poor lil' right one after I put on about 10-20 lbs over my high school/college weight.
posted by Stormfeather at 1:57 PM on January 23, 2011

Anecdata: normal weight, right-handed, left boob is larger, as is my left foot, FWIW.
posted by chez shoes at 2:12 PM on January 23, 2011

Best answer: To answer your first question:

From Manning, Scutt, Whitehouse, and Leinster (1997):
There is a general clinical impression that breast asymmetry is directional, that is, the left breast tends to be larger than the right (Gray 1974; Schaeffer 1953), although some studies have found no directional trend (Loughry et al. 1987; Manning et al. 1996; Mcfller et al. 1995). Our sample of signed asymmetries had a small positive mean (11.83 ml), indicting a slight tendency for larger left breasts. However, this was not significant (one-sample t-test, t = 1.89, p = .058).
This is behind a paywall, but I can email you the entire article if you memail me your email address.

This article has a pretty good review of bodily asymmetry in animals. Larger left sides seem to be a female-typical trait possibly related to sex hormone development, while larger right sides seem to be male-typical. They found that females and males with larger left sides tended to perform better on feminine cognitive tasks, while females and males with larger right sides tended to perform better on masculine cognitive tasks. From the first paragraph:
Systematic somatic asymmetry is widespread in the animal kingdom. Levy suggested some time ago (Levy & Levy, 1978) that sex hormones might affect the growth of the two sides of the body differentially. She reported that in right-handers, the right foot was more often larger in human males, while the left foot was larger in females. The interest in this measure arose from the possibility that the brain hemispheres followed a parallel pattern. The original finding of an interaction between sex and direction of foot-asymmetry, however, has not been reliably confirmed (Pomerantz & Harris, 1980).
posted by quiet coyote at 2:30 PM on January 23, 2011 [1 favorite]

Best answer: Plast Reconstr Surg. 1987 Oct;80(4):553-8.
A study of volumes of the right and left breasts of 248 subjects was undertaken using biostereometric analysis. [...] Analysis did not confirm the generally accepted clinical impression of left breast volume dominance. Although a size difference in breast pairs was documented, neither breast predominated.

Ann Plast Surg. 1989 May;22(5):380-5.
A study of the volumes of the right and left breasts of 598 subjects was undertaken using biostereometric analysis. [...] Analysis did confirm the generally accepted clinical impression of left-breast volume dominance. These results are shown to be consistent with those of a previous study using 248 women.

Br J Radiol. 1999 Mar;72(855):236-45.
The methods were applied to measure breast volume in 15 healthy females aged between 22 and 44 years (mean 31.7 years; SD 8.2 years). [...] ANOVA indicates that there is no significant difference between the mean volume of the left and the right breast (p = 0.294). The mean volume of the left breast is 561 ml (95% confidence interval (CI): 553 ml, 569 ml) and the mean volume of the right breast is 567 ml (95% CI: 559 ml, 576 ml). [...] However, a significant interaction was observed between the volume of the left and right breasts in different subjects (p < 0.005).

Aesthet Surg J. 2010 Jan;30(1):44-50.
The present study was performed to measure anthropometric breast values in Turkish female students and compare them with those of women in other nations. [...] The study included 385 female undergraduate student volunteers between the ages of 18 and 26 years with no physical or developmental deformity and with a body mass index between 20 and 26. [...]
The mean breast volume was determined to be 407.2 +/- 263.6 cc. The mean values of the right and left breast volumes were calculated as 415.2 +/- 264.5 cc and 399.1 +/- 265.5 cc, respectively; the right breast volume was significantly greater than the left breast volume (P < .001).

Anecdote: According the game Trivial Pursuit, in chickens, the left breast is generally larger. A quick search doesn't yield confirmation of this statement, though.
posted by lioness at 2:50 PM on January 23, 2011 [1 favorite]

Interesting. I'm a lefty and my left breast is about double my right!
posted by duddes02 at 4:21 PM on January 23, 2011

I'm a righty and my right breast is larger, and my right foot is larger than my left.
posted by Shirley88 at 5:30 PM on January 23, 2011

I'm ambisinister and my left breast is slightly larger than my right. Or at least they are when i'm not nursing; when I'm nursing, the more-recently-fed-from boob is always smaller.
posted by KathrynT at 8:04 AM on January 24, 2011 [1 favorite]

Also, if men's balls weren't lopsided, I reckon they'd walk funny.

amanda, not true. Balls hang at different heights (men "dress right" or "dress left"), which keeps us from playing a devastating game of knockers as we walk.
posted by IAmBroom at 8:17 AM on January 24, 2011 [1 favorite]

The entire right side of my body is slightly larger. Bigger breast, bigger foot, bigger hands. Bigger lung, even!
posted by Jilder at 10:29 AM on January 24, 2011

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