It is no surprise that female enrollment in college has increased over the last half century. The later age of marriage and childbearing and the greatly increased job opportunities of women explain the trend. Another factor, stressed by Becker in his pathbreaking economic analysis of the family, is increased emphasis on quality rather than quantity of children; parental education is an important factor in the quality of children.
The fact that women tend on average to get better grades in college helps to explain their lower dropout rate, but this is nothing new; even in the era when women dropped out of college to marry and have children, they had higher grades than men. That women are better students than men is pretty much a constant--and a puzzle.
When one observes members of one group outperforming another in a competitive environment in which, therefore, substitution of inputs is possible, a possible explanation is discrimination against the members of the superior group. If a college wants to have the the best students it can attract, and the women attending the college have better grades than the men who attend it, something is wrong--the school could increase the quality of its student body by admitting more women and fewer men. That it does not do so may be because it values other gender-dependent factors--for example, female students may prefer a lower ratio of female to male students than a purely meritocratic admissions policy would produce, and this preference may influence the college's admissions decisions. But this is unlikely to be a good explanation for the superior female academic performance today. The incentive to discriminate against female college applicants was much stronger in the old days, yet the female-male performance gap has not (so far as I can discover) diminished.
Women might outperform men academically because they worked harder, and they might work harder because they had more to gain from completing college successfully and doing so with high grades. But as Becker points out, since male participation in the labor force continues (and probably will continue) to exceed that of women, and since there is a large wage premium for college graduates, men actually have more to gain from completing college than women do. Yet not only do they drop out at a higher rate; but male college enrollment has not increased nearly as rapidly as female college enrollment has. Women are not just catching up with men on the educational front; they are becoming better educated than men.
So there are two puzzles: why women get better grades than men, and why men have a lower elasticity of response to the effect of education on earnings than women do. At this stage of our knowledge, the answers to these questions must be highly speculative; what follows, then, is guesswork.
The first question is, though, I think, a little easier than the second. From the standpoint of most teachers, right up to and including the level of teachers of college undergraduates, the ideal student is well behaved, unaggressive, docile, patient, meticulous, and empathetic in the sense of intuiting the response to the teacher that is most likely to please the teacher. Those are traits less characteristic of boys than of girls. Moreover, there is more variance in IQ among boys than girls--to exaggerate, more morons and more geniuses--and both the morons and the geniuses are difficult for most teachers, the morons for obvious reasons, the geniuses because they are easily bored in a class geared to the comprehension of the average student. So girls are easier to teach, and so are "rewarded" (not deliberately) with higher average grades.
Nothing in the suggested answers to the first question, however, can explain why males should be less responsive to the growing value of a college education than females. One possibility is that there is nothing more that men can do to improve their academic performance, given genetic limitations. Notice the curious fact that the more men in the lower tail of the male IQ distribution drop out at some stage in their academic career, the higher the average grades of the men who remain school should be; the "genius" tail pulls up the average, while the "moron" tail, being depleted because of dropouts, pulls it down less than it would if the students in that tail did not drop out disproportionately and thus cease to figure in the determination of grades. Maybe the "genius" tail, because of the publicity that its members attract, has obscured the fact that women may on average be more intelligent, or at least have innately a suite of qualities more supportive of academic perfornance, than men. The key is "innately." If aggressiveness and other psychological or cognitive qualities that inhibit male academic performance are innate, men may have maxed out long ago, while women did not reach their peak then because of factors extraneous to ability, such as lack of demand for women in high-skilled jobs, until recently.
Another possibility is that the decline of the conventional "patriarchal" family since the 1960s has been harder on boys than on girls. Because of rampant divorce and illegitimacy, a boy's biological father is less likely to be a continuous presence during the boy's formative years, and this is only one factor in what appears to be a decline in the disciplining of children. If docility is as I have suggested a factor in academic performance, a decline in discipline is more likely to harm the academic performance of boys than of girls because the former need more discipline to instill docility in them. It is difficult to test this hypothesis empirically, however, because grade inflation bedevils any effort to use changes in average grades over time as a measure of the trend in academic performance.
But, to repeat, these suggested answer to the puzzle of the gender education gap are highly speculative--a stimulus (I hope) to further thought, not the end of the inquiry.