Sex differences in intelligence: A multi-measure approach using nationally representative samples from Romania

The current paper investigated mean and variance sex differences for six cognitive ability measures across six nationally representative samples from Romania (between 1101 and 4000 for each measure, totaling 15,327 participants). The results show that fewer than 10% of the comparisons exhibit mean sex differences. Even when mean differences were observed, the effect sizes were mostly small and they were not replicated across measures. A very similar pattern emerged when examining sex differences in variance.

The random and non-replicable pattern of differences observed in the current research seems to support the conclusion that any sex mean or variance differences are likely spurious and the result of sampling or measurement errors than substantive and stable effects. This conclusion is supported for both general intelligence and second-level (more specific) abilities (e.g. performance vs. reasoning, verbal vs. performance, fluid vs. crystallized).

Naturally, the study is not perfect, but hopefully it will spur other large multi-measure studies:

The current study has a number of limitations. First, even though all the 6 samples on which we report data are carefully selected nationally representative samples, they are not comparable in volume to some of the samples on which data was reported in other studies, such as Deary et al. (2003), or Lohman and Lakin (2009). Therefore, while they make an important contribution for an understudied culture, they may only have a limited impact on the international state of knowledge. Second, some of the tests used in the current research were developed to be as sex neutral as possible. At least for the WISC-IV and SON-R, item bias was examined both by trained judges and through item analysis, and the GAMA and MAB-II were developed with the clear objective of minimizing adverse impact by gender. This may have affected the results and contributed to our null effect conclusion.

(Sadly, the paper is behind a paywall. If you want a copy, PM me.)