I’m trying to think of all my married friends everyone but in one couple the female married a male a year or so older thus she is.
One friend married a guy six months younger than she, but they were in the same HS graduating class, born the same year.
My dad is five years older than my mom. When I was little, I assumed all moms were younger than dads.
My husband is 3-1/2 years older than I am.
The author tried to isolate the demographic factor by analyzing groups with a homogeneous culture and little “marriage out.” The comparison of Orthodox and Hasidic Jewish marriage customs was very illustrative.