If the U.S. Census Bureau has it right, the 300 millionth American entered the United States kicking and screaming on Tuesday morning. The odds are that this milestone American is a boy, born to a white family in a California suburb. He will have a 1-in-4 shot of graduating from college, will probably marry, father two children, struggle with his weight, and live to see his 85th birthday.

What he will probably not have -- that his grandfather likely did -- is a pair of blue eyes.

Once a hallmark of the boy and girl next door, blue eyes have become increasingly rare among American children. Immigration patterns, intermarriage, and genetics all play a part in their steady decline. While the drop-off has been a century in the making, the plunge in the past few decades has taken place at a remarkable rate.

About half of Americans born at the turn of the 20th century had blue eyes, according to a 2002 Loyola University study in Chicago. By mid-century that number had dropped to a third. Today only about one 1 of every 6 Americans has blue eyes, said Mark Grant, the epidemiologist who conducted the study.

Grant was moved to research the subject when he noticed that blue eyes were much more prevalent among his elderly patients in the nursing home where he worked than in the general population. At first he thought blue eyes might be connected to life expectancy, so he began comparing data from early 20th- century health surveys. Turns out it has more to do with marriage patterns.