A recent study by the American Association of University Women suggests women earn only 80 percent of their male counterparts a year after graduating from college — a figure that drops to only 69 percent after 10 years.

In fact, the AAUW claims that the pay gap is larger for college graduates than the population as a whole, and it blames a part of the pay gap on sex discrimination.

Before women in the graduating class of 2007 become discouraged about a lifetime of unchecked sex discrimination and substandard wages, they should apply some of the critical thinking skills they learned in college and question the AAUW's claims.

First, sex discrimination is clearly illegal under the Equal Pay Act and the Civil Rights Act. If the AAUW has found cases where women make 69 percent or 80 percent of what men make with the exact same credentials and experience, it has uncovered rampant and widespread illegal activity.

Would thousands of organizations really engage in that much blatant, illegal discrimination and expose themselves to litigation where they would clearly lose? Not likely.

In fact, there are many differences between male and female workers that can explain differences in pay, and they have nothing to do with discrimination. They have everything to do with motherhood, marriage, career choices and time spent working.

College-educated women work an average of 42 hours a week, compared to 45 for men. In other words, men spend 7 percent more time on the job than women, and more hours of work translates into more pay.

Ten years after graduating from college, almost 40 percent of women are either out of the workforce completely or working part-time, compared with only 3 percent of men. Over a lifetime, men spend less than 2 percent of all potential work years out of the workforce, while women spend 15 percent of time away from work — mostly due to child-rearing.

When these mothers interrupt their careers and then return to full-time jobs, they naturally earn less than they would have if they had worked continuously, and it can't be blamed on discrimination.

Those realities might explain why college-educated women tend to go into fields like education and the humanities, which typically pay less than careers preferred by men such as engineering, math and business, but are also more conducive to long gaps of unemployment without depreciation of skills and the accompanying wage losses.

Voluntary career choices can make a huge difference in earnings that have nothing to do with discrimination. The average starting salary of liberal arts majors — which includes fields identified by the AAUW as preferred by females — is $31,000, approximately 40 percent below the average starting salary for majors in engineering, business and computer science.

Since women are disproportionately represented in majors like education and humanities that pay less, and men are disproportionately represented in fields like business and engineering that pay more, it would be natural for significant salary differentials to exist that have nothing to do with discrimination.

The economic reality is that marriage and motherhood explain almost all of the pay gap. For example, there is no pay gap among single, full-time workers between the ages of age 21 and 35. And never-married women in their 30s who have worked continuously earn slightly higher incomes than their male counterparts.

Simply put, here is what the AAUW didn't report: Median annual earnings of men and women age 25 to 34 with bachelor's degrees in the same field are roughly equal. In other words, there is no "pay gap," once you control for all factors that affect earnings, and compare apples with apples.

The female class of 2007 can enter one the strongest national job markets for college graduates in years, with confidence and optimism, and without any fear of widespread sex discrimination. There really is no pay gap once you properly control for all of the factors that affect earnings.

Mark J. Perry is a professor of finance and economics at the Flint campus of the University of Michigan. Reach him at mjperry@umich.edu.