Where are the male students? Colleges are increasingly worried about the way their applicant pools and student bodies are lopsidedly female. Much of the discussion assumes that the problem (if it’s a problem) is relatively recent.

A new study from the National Bureau of Economic Research, however, suggests that the enrollment patterns colleges are seeing today result from much longer term shifts. In fact, the analysis -- by three Harvard University economists -- suggests that but for certain societal conditions that either favored men or motivated men, the gap might have been present or larger earlier.

The study starts with a review of the long-term trends in gender enrollment and notes a fact that has received relatively little attention of late: Between 1900 and 1930, male and female enrollments were roughly at parity. And relatively few of the women enrolled (about 5 percent) were at elite women’s colleges. About half were at public institutions.

Citing a range of studies, the Harvard economists suggest that women of that generation -- like women today -- made calculated decisions about the gains that would come from higher education. Significant numbers were seeking careers, even with the knowledge that careers and marriage were viewed as incompatible both by would-be employers and would-be spouses. Others were seeking to marry college-educated men.

A variety of factors led to the relative growth in male enrollments in the following periods. Significantly, those changes did not reflect better academic preparation by men or any falling off in college preparation by women. Among the factors cited were the increase in bans on married women working, the importance of the GI Bill as a source of funds for college for veterans -- the vast majority of them men -- returning from World War II, and the desire of a subsequent generation of men to avoid the Vietnam War draft by enrolling in college.

Looked at through this historic perspective, the edge that men had for many years wasn’t natural or based on academic achievement, write the Harvard economists. They call their study “The Homecoming of American College Women,” driving home the point that the trends of today reflect a return of women, not the emergence of women’s outstanding academic performance.

The high point of gender imbalance in favor of men came in 1947, when men outnumbered women on campuses by a 2.3 to 1 ratio (a far more lopsided imbalance than we are seeing today, when women make up 57 percent of enrollments nationally). Women achieved parity again around 1980 and their proportions have since been growing. In terms of women’s motivations, the arrival of the women’s movement certainly played a factor, the authors write, as more careers were open to women and women delayed or opted against marriage and/or having children.

So why today’s imbalance? The Harvard economists suggest several factors. One is that changes in societal values have meant that more women -- across social classes -- hold jobs for significant portions of their adult lives, or their entire adult lives. The wage differential between college-educated and non-college educated woman has always been greater than that for men, the authors write. Women are behaving with economic logic by focusing more on college, since they will spend more of their lifetimes working.

The other major factor they cite is also very simple: Women do better in high school. They are more likely to study hard, to take the right courses, and to do well in those courses than are their male counterparts. Male high school students are more likely to have behavioral problems.

As a result, the authors suggest, today's gender gap really isn’t surprising.

An abstract of the report is available on the National Bureau of Economic Research's Web site, where the full report may be purchased online for $5.

The authors are Claudia Goldin, Lawrence F. Katz, and Ilyana Kuziemko.