By analyzing data from the 2000 U.S. Census, Nicholas Wolfinger and colleagues showed although male faculty are 21 percent less likely than male doctors to have a baby in their households, female faculty are 41 percent less likely than their female physician counterparts to have kids.

The researchers conclude that because it takes longer to achieve the job security of tenure in academics -- on average, professors achieve tenure at close to 40 years of age -- gender equality in the "ivory tower" has come at a cost.

"Many studies have examined the effects of childbirth on professional success, but few have considered how career choice affects fertility," Wolfinger said in a statement. "If women are sacrificing families for their jobs, the sexual revolution has not come nearly as far as might otherwise be expected."

The findings are scheduled to be presented this week at annual meeting of the Population Association of America in New Orleans.