The American name diversity index

Two ways to measure variety

The simplest measure is the share of babies given a top-10 name. In the 1880s that share was enormous; today it is a fraction of what it was. The second measure, the effective number of names, comes from the inverse Simpson index: it asks how many equally-popular names would produce the same spread you actually observe. It is a more robust figure because it accounts for the whole distribution rather than just the head of it.

What drove the change

Several things at once. Naming conventions loosened as the practice of reusing family names faded. Immigration broadened the pool. Mass media first concentrated attention on a few names and then, as it fragmented, scattered it. And parents increasingly treat a name as a way to mark a child out rather than place them in a line of descent — the exact inversion of what a name was for in 1880.

Why this makes rankings deceptive

It is the reason a modern number-one name is far less dominant than a historic one. When you read that a name is the most popular in America, the useful follow-up question is what share of babies actually received it, and the answer today is a much smaller slice than the same headline implied a century ago.