Conflicting views

Gur does not agree. “I don’t think their own data support their conclusions,” he said of Hänggi’s results. “With such a small sample, to claim that they can explain all the differences in connectivity by brain volume, and to do that by picking all sorts of subsamples, makes no sense.” Gur added that his group had covaried for brain size in their original analysis, finding that the sex effect remained. “It attenuated, but it did not go away,” he said of this secondary analysis, which the researchers did not include in the manuscript they submitted to PNAS.

UC Irvine’s Cahill added that even if Hänggi’s results stand up, they don’t negate the finding that there is a difference in wiring patterns in the brains of men, on average, compared to those of women, on average. “What is the single largest factor we have that distinguishes the big-brained and small-brained groups [in Hänggi’s study]?” he asked. “It’s sex. So even if we grant that Hänggi et al. are correct that brain connectivity differs according to brain size, we still have the same sex influence, on average.”

For Cahill, Hänggi and colleagues are operating under a false assumption of what constitutes a sex difference. Of course, some women will have bigger brains than some men, just as some women are taller than some men, but on average men are taller than women, and have larger brains. “It’s not a binary difference, like penis and vagina,” said Cahill. “That’s why I prefer the term ‘sex influence.’ Different brain size is a consequence of sex, so—directly or indirectly—the difference in brain connectivity is influenced by sex.”

But Hänggi maintained that brain size trumps sex in its influence on brain wiring. “Assuming that brain sex and brain size are highly correlated, similar effect sizes should have been reported in both studies,” he wrote in a follow-up e-mail. “However, the strongest effect size reported by Gur and colleagues, who investigated 949 subjects, was approximately d = 0.482. We investigated only 138 subjects, but we found effect sizes [for brain size] that were twice or three times times larger (d = 1.65).”

If Hänggi is right, and what was presented as a sex effect is indeed a brain-size effect, Fine noted that this finding would have implications for the work of neuroscientists trying to explain behavioral differences between the sexes. “Unless we have a reason to think that, due to their different brain connectivity, larger-brained men differ psychologically to smaller-brained men, and ditto for females, it no longer seems likely to be of profound behavioral importance that, overall, females and males have different patterns of connectivity,” she said.