Digit Ratios and Mutational Load

“Mutational load” is the idea that organisms contain some number of deleterious mutations. Some mutations will kill you outright, like the one for Tay-Sachs disease; some mutations greatly reduce your fitness but aren’t immediately lethal, like the inability to sweat; and some mutations are potentially problematic but mostly just kind of annoying, like colorblindness.

Random mutations happen all the time as a result of genetic transcription. The obviously bad ones tend to get weeded out of the population pretty quickly, but the ones with only a mild effect on fitness can stick around for a pretty long time. Under harshly Malthusian conditions where organisms compete for limited resources and danger and disease lurk at every turn, deleterious mutations will tend to get weeded out pretty quickly, but increase the food and decrease the danger/diseases, and a far larger % of your population will reproduce, including people who would previously have died.

One of the areas where mutational load seems to play a significant role is in IQ. I commented on a study n the subject back in “Is Genius Fragile?” While obviously a great variety of things go into determining one’s IQ, like whether you were in a good mood when you took the test and if your parents dropped you on your head as an infant, this particular study found that the major difference between extremely-high-IQ kids and normal-to-low-IQ people was that the normal-to-low people had a higher frequency of rare, slightly deleterious mutations. The lower the IQ, the more of these mutations.

Each mutation obviously has only a small effect–you could have several and still come out pretty smart. But to be one of the super smart kids, you had to basically be one of the lucky folks who escaped almost all of them.

IQ is interesting in another way: it is more variable in men than women. People make a big deal out of the greater preponderance of men than women at the very high end of the IQ distribution (especially math ability;) this is, we are frequently told, due to the pernicious evil effects of the patriarchy’s black-magic mind-control rays convincing women that they are bad at math. Strangely, however, we are never told that the opposite effect–the fact that the ranks of the intellectually retarded are also disproportionately male–is also due to the magical effects of the patriarchy.

BTW, if you think it is a problem that the evil patriarchy is preventing girls from getting math PhDs, but have no problem with boys being over-represented among the retarded, you are a horrible person.

No, it’s not the patriarchy. It’s the Y chromosome.

You see, because random unpleasant shit happens, like snake bites and random mutations, nature has built us with a fair amount of redundancy. If something happens to one of your eyes, you’ve still got the other. If something happens to one of your hands, you’ve got an extra. Etc. This is true on the genetic level, too, which is why you can survive even with small, fitness-reducing mutations.

But men have slightly less genetic redundancy than women, because they have an X and Y chromosome instead of two Xes. If a woman has a wonky mutation on one of her Xs, the other X may have a mutation that makes up for it. If a man has a wonky mutation on his X, his Y chromosome may have nothing to counteract it (and likewise, if there’s a wonky one on his Y, his X may have nothing to counteract it.)

Some mutations are good, some are bad, and some are neutral. Height is fairly neutral. The average man is taller than the average woman, but the spread from tallest men to shortest men is bigger than the spread from tallest women to shortest women. All women tend to cluster closer to the female average than men; there are both more “short men” and “tall men” than “short women” and “tall women.”

Likewise with IQ; there are both more male geniuses and retarded than female geniuses and retarded, most likely as a result of men having lower genetic redundancy to counteract the effects of mutational load.

On to digit ratios!

SlateStarCodex recently posted the results of the SSC/Less Wrong survey, which included digit ratios.

(To measure your digit ratios:

1. Place your right hand firmly on the plate of a photocopier or scanner with fingers straight. Close cover of place a sheet of paper over your hand to prevent glare from overhead lights. Ensure that the bottom crease and finger tip can be clearly seen in the photocopy.

2. Use a ruler or calipers to measure the distance from the middle of the bottom crease to the tip of the finger to the nearest hundredth of a centimeter.

3. Once you have the measures for both your ring and index finger, then divide the length of your index finger by the length of your ring finger. The result is 2D:4D (2nd digit divided by 4th digit).

If possible, please give three digits – for example, 0.915. Some people may have digit ratios slightly greater than 1, which is okay.)

Inspired, the husband and I decided to measure ours, too. Since we didn’t have a photocopier on hand, and were lazy, we just used a common tape measure. We measured both hands and checked each other’s work, but both of our hands came out identical.

I got a ratio of 0.971, he got 0.957.

(Note that the closer the ratio is to 1, the closer your fingers are to being the same length. The further the ratio is from one, the further apart your finger lengths are.)

Scott notes that the average male digit ratio in his survey was 0.972; the average female digit ratio was 0.975.

According to Wikipedia, a study of 136 males and 137 females at the University of Alberta found:[17]

  • Males: mean 0.947, standard deviation 0.029.
  • Females: mean 0.965, standard deviation 0.026.

People have taken to calling lower digit ratios (further from one) more “masculine,” and higher digit ratios (closer to one) more “feminine.” Which leads to the question of why all of these Rationalist math-nerds, whose community is definitely majority male and whose field is regarded as a stereotypically “Male” thing, should all have such overwhelmingly girly hands.

My first thought was that math nerds are effeminate. Which they are, for certain definitions of effeminate. But mathy women tend to be kind of masculine, which isn’t what this data shows. My second thought was that femininity/masculinity may be additive rather than subtractive–that is, having an extra unit of “masculinity” doesn’t necessarily mean someone must therefore lack a unit of “femininity” in a directly linear fashion. Some people could be very low in both femininity and masculinity, or high in both.

My third thought was that maybe measuring digit ratios is too complicated by measurement error and bias and random noise due to things like “how do your fingers crease?” and “did you actually use a copy machine?” A LOT of social science research doesn’t replicate at all.

My fourth thought was that a large difference between one’s finger lengths sounds a lot like physical asymmetry–which is caused (among other things) by mutational load.

Symmetry has long been recognized as one of the things people look for in a mate. Asymmetric faces (and bodies) are deemed less attractive than symmetric ones. Symmetry is a sign of good health, good lifetime nutrition, few parasites, and low mutational load. Asymmetry is a sign of things gone wrong.

Men display more of the effects of mutational load due to their Y chromosomes, so we’d expect to see a wider range of male digit ratios than female ones–which is indeed what the Alberta study found. Really dumb men probably have very different digit lengths, while really smart men trend toward even fingers. Women, because of their two X chromosomes, are probably just less likely to have really uneven fingers (just as they are less likely to be really dumb.)

The Slate Star Codex and Less Wrong cohorts, on the other end of the spectrum, are very smart people in whom we would expect to see lower mutational load.

The latest study I read on autism found that sufferers have a higher mutational load than the background population; while such an explanation is less fun than “autistics are secret math geniuses,” it is sensible. At any rate, if so, we should find a correlation between autism and divergent digit ratios, which the SSC/LW survey did. (Why autistics tend to be male should be immediately obvious.)

Likewise, if homosexuality is caused by some kind of genetic or parasitic agent, we would expect it to correlate with digit divergence. According to the Wikipedia, lesbians have more divergent digits than heterosexual women, but the jury is still out on gay men.

Interestingly, Wikipedia reports that the Han Chinese (who score very well on IQ tests,) have very even fingers, and that the Jamaicans (who do not do so well on IQ tests,) have very divergent ratios. (However, like much of this digit ratio research, I regard this as speculative.)

Of course, like height, there my also be an androgenic effect, such that men are supposed to (for whatever reasons) have slightly different digit ratios than women. After all, even the SSC/LW sample had more divergent ratios for the men than the women, even though the whole SSC/LW population probably has about equal mutational loads (having been pre-selected for high IQ, which = low mutational load.)

 

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