Thanks for all the responses, this has been a very interesting discussion to follow. There has been quite a variety of explanations put forth for the male bias on iNat, and while I find some of the reasons more plausible than others, I expect the answer is complicated and involves many of these and probably more. But I think (as others have already suggested) that the real reason for the discussion is to explore ways that women (and other minority groups) can feel more welcome in the community. It’s somewhat telling to see that although most of the voices in this thread are male, the “likes” on posts suggesting we investigate the issue further are more female-biased.
I looked through users who recently signed up to iNat for ones whose gender seemed clear from the name and/or photo. Of the 60 I looked at, about half were female. If the sign-up rate is approximately even (though a larger sample size would help clarify that), then something is happening once people are already on the platform. This could be, as others have said, a case of personality or interests aligning more for males than females, but there could be more to it. And even if it’s a personality/interest thing, I still feel iNat could have a positive impact on female retention by making some changes. For example, if the leaderboards are more appealing to men, maybe make them less visible on the site. I also looked at the last year of Observations of the Week and found the skew at about 70% male. Given that the top observers are about 80% male, this could be interpreted as potentially favoring women, but I wonder if it could be further improved. (This is not a criticism of @tiwane.)
So I would suggest that rather than further discussing if there is a male bias and whether it matters, I think we should move on to brainstorming ways to be more inclusive for everyone. What are concrete changes that iNat can make to attract and retain a diverse user base?