Are smarter people (a bit) more symmetrical? A matter of how to adjust for publication bias?
Variation in general mental ability (g) may be due to a general fitness factor, leading to the prediction that g should relate to indicators of fitness. One such an indicator may be fluctuating asymmetry (FA), the morphological outcome of developmental instability. But the general association between FA and fitness components has been debated for several decades. Therefore, the meta-analysis by Banks et al. (2010) on the association between FA and intelligence is very timely and relevant. However, I argue that Banks et al. (2010) did not take the opportunity to fully address the possible issue of publication bias thoroughly enough, providing no estimates of average effect sizes adjusting for publication bias. In doing so in this paper, I show that adjusting for publication bias leads to markedly lower average effect sizes that were in some cases no longer statistically significant (between â€“0.06 and 0.12 vs. 0.14). I emphasize though that, at
present, the assumptions behind the use of the corrections for publication bias cannot be tested explicitly leading to the, perhaps, disappointing conclusion that in spite of the 14 samples across nearly 2000 individuals, it is at present impossible to come to any robust conclusions. The evidence in favour of publication bias presented here, however, arguably suggests that associations between FA and g are at best very weak, but may also be non-existent. Further research, based on larger samples in combination with unbiased reporting, is undoubtedly indispensable to come to robust conclusions about the association between FA and intelligence.