They have a safety net, they take less risk. If she has one dim X chromosome, she has another one to fall back on which would make her smarter than her idiot brother. The other side of that is if she has a bright X chromosome, it also get mitigated by the other X making her less brillant than her genius brother. That's why women are more likely to be average in intelligence to each other and stable. Men provide the wide swings.
Women provide the stability in the movement of humankind.
Without women, humans might have anniliated themselves before we ever got a chance to get into mudhuts. For better or for worse, the civilization we have is the result of both genders working in tandem. Men do have a higher percentage of geniuses for instance. The other side of that is they have a higher percentage of autistics, mentally retarded, schizophrenics and criminal sociopaths. Intelligence is strongly influence by a gene on this chromosome.