What is the Most Common Gender Distribution for Families?

Among parents with four children, what is the most common distribution of boys and girls? My friends think it’s two of each sex.

—M.E., Mesa, Ariz.

Your friends are wrong. Having three children of one sex (and one of the other) is most common. These are the possibilities (sexes noted in birth order):

(1) BBBB (2) BBBG (3) BBGB (4) BGBB (5) GBBB (6) BBGG (7) BGBG (8) GBBG (9) BGGB (10) GBGB (11) GGBB (12) GGGG (13) GGGB (14) GGBG (15) GBGG (16) BGGG

So, families with three children of one sex can occur in eight different ways (or 50 percent of the time), two of each sex can occur in six ways (37.5 percent) and four of the same sex can occur in two ways (12.5 percent).