Read between the lines

Here's a good example of a research tool so accessible, so appealing, that scientists are clamoring to put it to use. From University of Bath professor Mark Brosnan, via this month's Atlantic:

The digits under investigation are the second digit (2D – next to the thumb) and the fourth digit (4D – next to the little finger). 2D divided by 4D (2D: 4D ratio) provides an index of exposure to testosterone while in the womb. The HOX gene family is required for the growth and patterning of digits and the differentiation of the genital bud. Hoxd and Hoxa genes are strongly expressed in the gonads and are also required for the growth and differentiation of digits. This sharing of causal factors in digit and gonad differentiation allows patterns of digit formation to be a marker for prenatal sex hormone concentration (Manning et al., 1998)

This was surprising to me -- the hormones we're exposed to in the womb affect our finger lengths in statistically significant ways. OK. It's tempting to wonder if our prenatal environment can leave other lasting marks:

Prenatal testosterone slows the growth rate of the left side of the brain while enhancing growth of the right side (Geshwind and Galaburda, 1985). The right hemisphere is associated with better visual-spatial and mathematical abilities, as is the 2D: 4D ratio (Manning and Taylor, 2001). Handedness may also be affected (Manning et al., 2000). Thus, traditional sex differences in visual-spatial and mathematical abilities can be attributed to differences in exposure to prenatal testosterone, indexed by a sex dimorphic pattern in digit ratio.

Bronson goes on to try to show that prenatal testosterone exposure, as assessed by digit ratio, is correlated with science department membership at a university. I know, I know -- most people would never think to examine such a relationship. Yet this kind of intellectual leap is common inside academic departments! Either these people are really bright or really self-absorbed.

As the levels of testosterone decrease in males, performance upon visual-spatial measures increase whereas comparable cyclical declines in female testosterone result in decreased performance upon visual spatial measures (Moffat and Hampson, 1996; see also Sanders et al., 2002). Thus those with the greatest visualisation skills will be females with the lower male-typical digit ratio (0.98) and males with the higher female-typical digit ratio (1.0). Additionally, an extreme digit ratio (0.94) has been associated with Autism, suggesting that excessive prenatal testosterone may play a causal role in the development of autism (Manning et al., 2001). Using some of the visual-spatial measures described above, Dyslexia has been cast as the opposite end of the continuum to Autism (Brosnan et al., 2002).

So interesting: High testosterone exposure in men leads to low index-to-ring ratio (relatively small index fingers) and, also, poor visiospatial skills. There's a few wrinkles -- high T seems to improve visiospatial skills in women, and really high T seems, cyclically, to strengthen visiospatial skills in men.

Armed with this knowledge and some calipers, Bronson's team measured the faculty -- focusing on the science and engineering types, and comparing to the humanities and business school types:

…An ANOVA was conducted to statistically identify whether there were differences between different groups (sex, faculty or pay scale) that related to differences in digit ratio... This shows that there was a significant difference between the faculties in digit ratio (F=3.519, p=0.034). A Tukey HSD post hoc analysis revealed that the significant difference was between the Science faculty and the HASS/Management faculty (p=0.02), with the Engineering faculty averaging between the other two faculties.

Brosnan's data is not yet in publishable form, so I imagine we'll eventually see histograms or breakout data sets by gender. It might clarify this link, or totally eliminate any relationship (one physicist with Asperger's could skew everything!)

But before you go lining up your fingers, kind readers, note that the ratios Brosnan is talking about have a variance of about 5%. So, a mismeasured millimeter or two could lead you to abandon your science career and become a testosterone-addled middle manager. Surely this is a choice we've all contemplated, but never with hard data to back it up. So be careful. And then go ahead and do what you've always wanted to, anyway -- confident in the knowledge that you are more than the lengths of your fingers.

Not surprisingly, there’s been finger-length controversy before, concerning the link to homosexuality. This might be a more intuitive link to examine than science department faculty membership, but ultimately, just as muddled and equivocal.

Someday, this line of thinking might lead to better predictors of future careers, better explanations for our desires and preferences. Until then, we muddle through weak correlations and heavy overlap. But cocktail party science marches onward! Next up: Thoughts from the future influence random number generators.