Names trend over time in rather smooth curves of popularity.

In the U.S., there aren't any laws about what you can call your kids, but the Social Security Administration tracks popularity of names. For instance, the second most popular girl's name this year is Emma, which was also the third most popular in 1880 ... and the 451st most popular at its low point in 1978. The most popular name today, Sophia, tracks a similar curve with a low point in the '40s.

The most popular girl's name in my age cohort was Jennifer — the #1 girl's name from 1970 to 1984! — but Jennifer has been on the way down ever since. Today's American girls are more likely to have an Aunt Jenny than a classmate Jenny. To me, Jennifer (or Jessica, Melissa, Amy, or Heather) sounds like someone my age, not a little kid. Young girls are named Ashley, Hannah, Madison, Alexis ... and baby girls are Isabella, Sophia, Emma.

Male names are stabler than female names, but mostly because some names (Michael, Matthew, Daniel, William ...) are persistently popular.