Origin of the fe/male brain myth
When scientists present their result in a scientific paper they are trying to convince the reader of the truth of a specific finding. That means they present their data in the most convincing manner rather than the most truthful manner. So when scientists are trying to prove that male and female brains are different they almost exclusively present only averages over a large number of individuals rather than the whole distribution because this looks "cleaner" and makes the finding they are presenting the obvious to the reader (and hence more easy to publish). However this can also cause unsavy readers to think the differences are more important than they really are.
For instance this paper finds men have significantly bigger brains than women by approximately 90 cm^3 (or ~8% difference) which sounds like a lot. But this is just a difference in population means, if we look at the actual distributions of brain size for women and men in that paper (see below) it's obvious there is a lot of overlap so inferring the existence of a "male brain" and a "female brain" from this data makes as much sense as saying there exists a "male height" and a "female height".
Now that I've mentioned height let's look at another paper which finds significant difference in brain size between genders across all ages. They have a very convincing looking figure (see below left) to support this claim. But what is not obvious from the figure is that these are averages once again representing a total of 2,773 males and 1,963 females. This paper helpfully did the same analysis looking at the height of these individuals (below right) which makes it apparent that the gender-differences in brain size are no bigger (and may actually be smaller) than gender-differences in height.
Every single study I've come across finding differences in brain size/morphology or behaviour between men and women follows the same story. Using a study population more than 1,000 they find highly significant differences in the mean of characteristic X on the order of 3-10% (p < 0.05). Which in the discussion they then link to current sexist stereotypes (ie. that women are better at multitasking & emotion and men are better and logic, math, science). But if you figure out the population distributions men and women overlap for >80% of the range of values observed - meaning that it would be impossible to predict the gender of a person on the basis of the brain/behaviour pattern.
The New Study
Okay so now that we've established that scientists have known all along that you can't predict a person's gender from any individual brain/behaviour characteristic we can get to the current study. This new study tries to address the possibility that while each individual characteristic might not be a good predictor of gender perhaps they all correlate together so that "male brains" & "female brains" are cumulatively quite different from each other even if each individual difference is quite small and meaningless. This is similar to the difference between wolves & coyotes - there isn't a single distinguishing characteristic between the two rather several characteristics are combined to tell them apart. If this is the case then brains should be fairly consistent possessing all the male-ish characteristics and no female-ish characteristics or all female-ish and no male-ish characteristics.
This new study finds little consistency within brains, with the majority having various combinations male-ish and female-ish traits and only a tiny fraction (<10%) showing consistent male-ness or female-ness. Strikingly this was even true for the single behavioural study which finds gender to be a meaningful way to group individuals (as long as they are US undergraduate psychology students). That analysis shows that knowing someone's gender will typically let you correctly guess their interest (or lack of interest) in ~5 of the 10 most sexually-dimorphic activities.
All together this shows that gender is probably not a key axis of variation for either brains or behaviours and that the concept of a "male brain" and a "female brain" is meaningless. Hopefully this will mean these fields can move beyond gender to look at other potentially important axes of variation (eg. culture, genetics, geography).
