For most of the nation's history, there have been more males in the U.S. population than women. But early in the 20th century this ratio began to shift. Now there are more females than males.

Certain developments and historic events were consequential in shifting the proportions of one sex to another.

• The Civil War and World Wars I and II wiped out the overabundance of men, creating an overabundance of females.

• Between the Civil War and World War I, however, men became preponderant again.

• Then, maternal deaths in child birth -- thanks to medical advances -- fell from 608 per 100,000 live births in 1915 to a minuscule seven per 100,000 in 1995.

• Immigration -- which is usually disproportionately male -- was curtailed in the first half of the century and did not increase until the 1965 Immigration Act.

From nearly 108 men for every 100 women ages 20 to 39 in the 1920s, the ratio sank to roughly 95 men for every 100 women of those ages by 1970. Since 1970, the proportion of men has rebounded to a slight excess of men in the 20 to 39 age group. But among men of all ages the rebound was weak and still left women in the majority.

It appears to demographers that about three-quarters of the post-1970 rebound in males age 20 to 39 was due to the increased immigration.

Source: Peter Brimelow, "The Missing Men," Forbes, March 20, 2000.