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from Luders et al (2005), [click for full caption]Luders et al (2004) used a 3D MRI technique to measure cortical folding (cortical complexity) on 30 men and 30 women. Luder's group suspected that women's smaller cranial size would necessitate more cortical folding -- additional folding meaning additional surface area, which could offset the possible disadvantage posed by lower brain volumes. Luder found more gyrification in women's frontal and parietal areas. Men did not show greater complexity in any lobar region.
Luders et al (2005) later got attention with another MRI study showing a proportionally thicker cortex (and greater proportion of grey matter) in women than men. Im et al (2006) confirmed this finding, but points out that women's cortexes are thicker in specific areas, spread across the cortex. Sowell et al (2006) found that women had a thicker cortex in the right inferior parietal and posterior temporal regions, even without correcting for overall brain size.
In contrast, see Haier et al (2005), whose well-publicized (MSN) results found more white matter in women than men "related to general intelligence."
I'd read about Haier's study, and heard a lot of people (largely of an evolutionary psychology bent) try to use women having more white matter as an explanation for differences in behavior. Except that Haier doesn't appear to be saying that women have more white matter, but instead that the difference is in how the brain regions are used. You could say that men and women are using different parts of their brains. I'd be more likely to say men and women are failing to use different parts, but I'm a cynical bastard. However, reading Haier's report, I'm not overly impressed by how he relates structure to general intelligence. Basically, he appears to be saying that women who do well on IQ tests tend to have more white matter than women who do poorly on IQ tests (substitute men and grey and repeat). I'm not convinced yet, but I could be. So convince me. Why is this a good test?