Attraction and Selection 1: Prologue & Symmetry


This biological soliloquy (in 3 parts) is brought to you by Pine Street Barbershop. More accurately, it started with my visiting said barbershop to request a trendy haircut.


I’ve worn my hair in an unruly mop for the last ten years, chopping back the thin curls when they entangle my ears. With male pattern baldness setting in, I decided a few weeks ago that I may as well try to look hip while my follicles still work.

Thus resolved, I marched into Oregon Hill and asked for an undercut. Lots of handsome people seem to have them, and anything that makes me more like Robyn is a good style choice in my book.

The buzzers sheared away the hair from back and sides of my skull, every lock I lost bringing me closer to cool. After paying the barber, I strolled over to the gym for my daily workout, pleased with how refreshing the breeze felt on my exposed temples. It wasn’t until I got to the weight-room floor that I realized something was horribly amiss. Whimpering through a set of dumbbell curls, I glanced in the mirror and beheld the sad truth:


There were 800 people at the gym with this haircut, but I can only be bothered to draw two in addition to myself. For real.

I had the same haircut as everyone else at the gym. More importantly, it looked way better on them.

Mostly because they were hot.

Being the scrawniest kid at the gym is not a new experience for me. I remind myself that people come in all shapes and sizes, and that there many ways to be beautiful. Like every other liberal, my Facebook feed is littered with friends posting progressive, body-positive articles, and I read a fair number of them. When I attempt to internalize the blogosphere’s collective encouragements to myself, a conversation like the this ensues:

Dan: It’s okay that I have creepy skinny bird limbs. That can be attractive. I’m still attractive.

Inner Biologist: Do you have any evidence to suggest that?

Dan: If I had the resources, I could recruit 100 volunteers and measure self-reported arousal–

Inner Biologist: Shut up, that’s stupid and gross. The data already exist. People like muscles.

Dan: What?

Inner Biologist: A man’s lean muscle mass is a strong predictor of his number of lifetime sexual partners [1]. People bonk people they find attractive. A trait associated with a better bonk-score is more attractive. This suggests your lack of muscle shouldn’t be considered attractive.

Therein rests the heart of the matter: I can’t bring myself to believe “attraction” as 100% culturally defined, or solely an expression of personal preference (although it is in part both of these things). I’m convinced–perhaps erroneously–that attraction is an expressed trait with at least some biological underpinnings. This trait has been shaped by generations of sexual selection [2], in which fit individuals correctly identified other fit individuals, swapped some genes (and body fluid), and produced fit babies. The babies that inherited their parents’ keen eye for quality genetic merchandise eventually hit adolescence, started feeling/expressing attraction, and made their own fit babies.

What about offspring who didn’t inherit such preferences and chose less-than-fit mates? They produced tragic babies with weird genes. Those babies got selected against, with Grim Selector (i.e., Charles Darwin with a scythe) showing up and removing them from the gene pool.

That attraction to lean muscle mass in men? It’s been selected for, and it’s here to stay.

The Grim Selector. Only distinguishable by the Grim Proletariat by hairline and reading material.

The Grim Selector. Only distinguishable from the Grim Proletariat by hairline and reading material.

Now, you could argue that lean muscle mass is only one trait, inferring some–but not all–adaptive benefits. You might want a mate that is intelligent or has a good immune system in addition to upper body strength. An organism’s fitness results from thousands of traits interacting with a given environment. Wouldn’t it be useful if there were one trait trait that signaled general fitness?

Turns out, there might just be such a trait for us bilaterally symmetrical organisms [3]! Organisms are rarely 100% symmetrical externally (even less so internally, but ignore that right now). Small, random perturbations from left-right symmetry are referred to as fluctuating asymmetry [4]. Prevailing theory holds that these deviations from bilateral symmetry result from the events and challenges an organisms faces during development.

Missed a few meals in the middle of a growth spurt? That shortage of nutrients could mean only your left arm bulks up. Lived in a crappy environment when your teeth were coming in? Better invest in a retainer or risk a snaggletooth smile (at least if you’re a shrew [5]). From parasite infections [6] to intolerable temperatures [7], an adult’s traumatic past can be writ small in asymmetries accumulated over their youth.

Of course, not every asymmetry indicates lower fitness. Additional examples of adaptive asymmetry here and here.

Of course, not every asymmetry indicates lower fitness. Some other examples of adaptive asymmetry here and here.

Some of these stresses may be random; an organism was just in the wrong place at the wrong time. However, other asymmetry-inducing stresses could mitigated by higher-quality traits. For example, more efficient foraging could lead to better feeding even in times of scarcity. Better dispersal abilities could move the organism away from a crappy environment. A stronger immune system could keep parasite levels low. And so on. Thus, fluctuating asymmetry might be an honest signal of mate quality, and organisms that recognize and select more symmetrical mates may pass on better genes to their offspring [8].

There is substantial empirical evidence for this hypothesis (and some against [9, 10]), but the seminal paper that everyone and their mom cite was published in 1992 on Barn Swallows (Hirundo rustica). A scientist by the name of Anders Pape Møller* conducted a study [11] in which he manipulated the symmetry of males’ long outer tail feathers (known as retrices in ornithological circles). Møller clipped the ends tips off, then re-attached them in a fashion that induced asymmetry through shortening/elongation, or maintained symmetry for controls. Møller then observed the reproductive behavior and success of these males throughout the breeding season.

The genus name for Barn Swallow comes from the Latin word for swallow "Hirundo", which also means "flying fish". The Romans weren't particularly with wildlife identification.

The genus name for Barn Swallow comes from the Latin word for swallow “Hirundo”, which also means “flying fish”. Romans weren’t particularly gifted at wildlife science.

The results? The asymmetrical males (particularly those made asymmetrical through shortening) took longer to attract a mate, had later & smaller clutches, and fledged fewer offspring compared to the more symmetrical controls. Females appeared to evaluate males by how symmetrical they were, even though their wonky tails didn’t affect flight performance. Moreover, females invested less reproductive effort if they ended up with a lopsided fellow.


A tough reality to swallow.

There are any number of morphological features that can be used to evaluate symmetry. I’ve observed variation in the charismatic face patterns in the population of Golden-winged Warblers I study. Some male’s bibs seem uneven, and others have white “beauty marks” below one eye but not the other. I’ve often wondered if females make mate decisions based on the symmetry of such features.

Screen Shot 2015-01-06 at 2.17.21 PM

Slightly asymmetric male Golden-winged Warbler from Highland County. Don’t get me wrong, I think this guy is gorgeous…but I’m not the one he’s needs to woo.

Speaking of faces and symmetry, let’s talk about Homo sapiens. Humans are face-centric organisms: we have an incredible memory for faces, and focus more on faces than any other feature. It’s not surprising that the symmetry of one’s face can influence how attractive one is perceived [12, 13, 14, 15, and a lot of other studies] or response to their bedroom performance [16].

My favorite paper on this topic was published by Little et al. in 2007 [17], in which scientists measured symmetry preference at different stages of women’s menstrual cycles and among women with different relationship statuses. The researchers found that women showed increased preference for symmetrical faces during peak fertility: the phase in which the genetic quality of the sexual partner mattered most.** Additionally, partnered women showed a much stronger preference for symmetrical faces than un-partnered women. The evolutionary hypothesis generated by this finding? If already partnered, a woman’s investment in an extra-pair fertilization is only worth it if the genes contributed are of high quality, or at least higher quality than her current partner. If un-partnered, it may be better to accept low-quality genes than to miss out on the opportunity to reproduce.***

Being attractive to women who are biologically primed to conceive isn’t my top objective right now. I lack the income to support a family, and I’m quite content with my current boyfriend. In spite of this, I compulsively compare my face against the men I encounter both in person and in the media. The resulting continuum does not inspire confidence.


If my boyfriend were geographically this close to Tom Daley, his expression would possess more predatory hunger.

But hey, symmetry isn’t the only signal of fitness. There are plenty of other traits that may elicit attraction, and my tiny arms and lopsided face may be outweighed by other features.

More on that in Part 2.

*I am aware of the controversy surrounding some of Dr. Møller’s research, particularly the heritability of asymmetry. The 1992 Nature paper described above, to the best of my knowledge, remains debated but un-retracted.

**Humans copulate for lots of reasons, like social bonding [18] or because the TV’s not working [19]. That being said, sexual behavior and attraction appears to change during peak fertility, both for the woman ovulating [20] and the men she interacts with [21].

***This verbiage is always troubling, as it implies that everyone wants a baby on some level, even if it’s not a conscious one. I write this post with the knowledge that human evolutionary behavior and sociobiology are fraught topics with famous critics. We are legitimately concerned with how our perceptions are shaped by culture, and how these perceptions generate bad hypotheses, ugly studies, and erroneous conclusions. So be it.


1. WD Lassek and SJC Gaulin. 2009. Costs and benefits of fat-free muscle mass in men: relationship to mating success, dietary requirements, and native immunity. Evolution and Human Behavior 30(5): 322-328.

2. P Brennan. 2010. Sexual Selection. Nature Education Knowledge 3(10):79.

3. Bilateral (left/right) symmetry. Understanding Evolution. <>

4. L Van Valen. 1962. A study of fluctuating asymmetry. Evolution. 16(2): 125-142.

5. AV Badyaev, KR Foresman, and MV Fernandes. 2000. Stress and developmental stability: vegetation removal causes increased fluctuating asymmetry in shrews. Ecology 81(2): 336-345.

6.  AP Møller. Parasites differentially increase the degree of fluctuating asymmetry in secondary sexual characters. Journal of Evolutionary Biology 5(4): 691-699.

7. TR Gest, MI Siegel, and J Anistranski. 1986. The long bones of neonatal rats stressed by cold, heat, and noise exhibit increased fluctuating asymmetry. Growth 50(3): 385-389.

8. AP Møller and A Pomiankowski. 1993. Fluctuating asymmetry and sexual selection. Genetica 89(1): 267-279.

9. RA Palmer. 1998. Detecting publication bias in meta-analyses: a case study of fluctuating asymmetry and sexual selection. The American Naturalist 154(2): 220-233.

10. T Bjorksten, P David, A Pomiankowski, and K Fowler. 2000. Fluctuating asymmetry of sexual and nonsexual traits in stalk-eyed flies:  a poor indicator of developmental stress and genetic quality. Journal of Evolutionary Biology 13: 89-97.

11. AP Møller. 1992. Female swallow preference for symmetrical male sexual ornaments. Nature 357: 238-240.

12. SW Gangestad, R Thornhill, and RA Yeo. 1993. Facial attractiveness, developmental stability, and fluctuating asymmetry. Ethology and Sociobiology 15(2): 73-85.

13. R Thornhill and SW Gangestad. 1993. Human facial beauty. Human Nature 4(3): 237-269.

14. K Grammer and R Thornhill. 1994. Human (Homo sapiens) facial attractiveness and sexual selection: the role of symmetry and averageness. Journal of Comparative Psychology 108(3): 233-242.

15. N Koehler, LW Simmons, G Rhodes, and M Peters. 2004. The relationship between sexual dimorphism in human faces and fluctuating asymmetry. ProcB 271: S233-S236.

16. R Thornhill, SW Gangestad, and R Comer. 1995. Human female orgasm and mate fluctuating asymmetry. Animal Behavior 50(6): 1601-1615.

17. AC Little, BC Jones, DM Burt, and DI Perret. 2007. Preference for symmetry in faces change across the menstrual cycle. Behavioral Psychology 76(3): 209-216.

18. RJ Levin. 2002. The physiology of sexual arousal in the human female: a recreational and procreational synthesis. Archives of Sexual Behavior 31(5): 405-411.

19. S Whitelocks. 2013. The blackout baby boom! Hurricane Sand to spark 30% rise in births this summer after October power outage left cupels with little else to do. Daily Mail 2013-Feb-28.

20. SW Gangestad and R Thornhill. 2008. Human oestrus. ProcB 275: 991-1000.

21. SC Roberts, J Havlicek, J Flegr, M Hruskova, AC Little, BC Jones, and DI Perrett. 2004. Female facial attractiveness increases during the fertile phase of the menstrual cycle. ProcB 271: S270-S272.


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