And when most traits and effects are like 50% genetic and 50% environmental, there's very little reason to believe this should be different for intelligence.
That actually varies wildly based on the trait: the more critical for survival of a specified trait, the less genetic variation there is (in terms of how it affects the phenotype. genes which don't affect the phenotype as strongly can drift more). Basically, strong selection pressures correlate to traits with low genetic variability. Ones where it's 50/50 tend towards "cosmetic" differences (i.e. they could be functional, but they don't affect your reproduction chances). So it can be argued that the genes for e.g. hair color are going to have experienced genetic drift far more than the genes for intelligence.
Biological sex really isn't that arbitrary of a dividing line, given the sexual dimorphism
What I meant was that calibrating things so there's equality is arbitrary. It's an ideological statement that there's no dimorphism in a specified trait, which is not based on scientific principles, but what we wish to be true. If you think about it, "geekiness" could probably be calibrated, then the population divided 50/50, and we come up with brain-tests that calibrate so that more or less geeky people are equally "intelligent". e.g. there are
definitely skills that less-geeky people can master much easier than the average "geek". Why do we automatically assume "geeks" are the smart ones, when there are a wide range of skills the average geek is terrible at compared to non-geeks.
The likely answer to that conundrum, is that there's no single "g factor" that makes sense. From what I've read, some researchers have managed to split cognition ability into ~3 separate skills, in a way that each one is completely statistically independent of the other two. If the "g factor" posited by IQ proponents really existed, such a split should be impossible. And there are definitely brain-related abilities that we're not measuring with "intelligence" tests at all, because IQ tests assume that cognitive symbol manipulation of some sort is the only sort of brain-ability that matters. What about people who have amazing reflexes and neuro-physical abilities? Aren't they smart too? Their brain might be optimized to process more sensory and motor skill data, but we're not measuring that: we're not taking into account differences in how each brain is optimizing it's finite amount of data processing.
What is much more promising is looking at the brain as making trade-offs between different abilities, and that some of these trade-offs are correlated with gender via itermediary systems (pre-natal testosterone is the prime suspect) so that is what allows IQ testers to calibrate a test that equalizes "g" between the genders.
Pre-natal testosterone research is really the promising lead here. It can explain the "expected" gender differences, but also why some people buck the trend. e.g a boyish girl could just be born that way, and isn't just a result of some fluke of environment. It's certainly a better fit for the data than saying you were just a certain way "because" you had a Y chromosome, or "because" you were socialized as a boy. Neither of those older theories can explain outliers very well: i.e if either "X/Y chromosome" or socialization is so completely overwhelming, why do some people turn out completely differently given the same inputs?