It's well known that for the SAT mathematics test, a) male high school students in the U.S. have higher scores on average than females, b) the gap is large and statistically significant (+30 points), and c) the male-female math test score gap has persisted over time, since at least 1971, and probably much longer (see chart above, data here from the Dept. of Education).
One explantion for the female-male math test score gap is summarized here by Janet Hyde et al.:
In 2007 the SAT was taken by 798,030 females but only 690,500 males, a gap of more than 100,000 people. Assuming that SAT takers represent the top portion of the performance distribution, this surplus of females taking the SAT means that the female group dips farther down into the performance distribution than does the male group. It is therefore not surprising that females, on average, score somewhat lower than males. The gender gap is likely in large part a sampling artifact.
MP: In other words, it is only because more females than male take the SAT exam that males score higher on average than females, and if the sample sizes were more equal, the difference in mean math test scores would disappear.
Consistent with this explanation of the difference in mean math test scores would be the following assumption:
Ceteris paribus, if the number of females taking the math SAT exam relative to males (and female percentage of total) increases over time, the male-female math test score gap should INCREASE over time, since an increasing number of females (and increasing percent of total) taking the SAT should lower female mean math test scores over time relative to male math test scores. Reason? The increasing number of females taking the SAT will "dip further down into the performance distribution" over time.
Using Census Bureau data, the chart below shows that females taking the SAT exam as a percent of the total increased from 50% in 1975 to 53.6%, as the male percentage has decreased from 50% to 46.4% over that period (see chart below).
According to the reasoning above, as the number of females taking the SAT exam increased over time (along with the percent of total) relative to males, the mean female score should have decreased relative to the male mean score, and the male-female gap should be INCREASING over time, theoretically.
But that is exactly the opposite of what has actually been happening. The chart below shows that the male-female gap has actually been decreasing over time, even as more females took the test relative to males, from a high of 46 points in 1977 to a gap of 33 points in 2008.
Bottom Line: The gender gap appears to be more than just a sampling artifact, since the decreasing male-female math test score gap is exactly the opposite of what the Hyde et al. hypothesis would predict.
Update: Additionally, if the number of females taking the test increases over time, the Hyde hypothesis would also predict a falling mean female math test score over time, when in fact we see the opposite: a rising female mean SAT math test score. Comments welcome.