Shirley Temple didn’t make many enemies, but Alleen Nilsen can think of a few people who loathed America’s sweetheart. Nilsen, a professor of English at Arizona State University and president (with her husband, Don) of the American Names Society, once met a Shirley from a family that used the name for four generations — for its men. As soon as Temple stamped it as indelibly girlish, Shirley IV disgustedly switched to Shirl. There was no Shirley V.

Dozens of longstanding male names — Kim, Beverly, Ashley, etc. — have met the same fate. Linguists know the pattern well: not long after a boy’s name catches on with girls, parents shy away from christening sons with it. “We crowd them out,” Nilsen says. Consider some examples from the Social Security Administration’s baby-name database. Through 1955, “Leslie” consistently appeared among the 150 most popular boys’ names. About a decade earlier, it began to catch on among girls. And the “crowding out” Nilsen mentioned took place. “Leslie” fell out of favor, dropping from a peak of 81 in male popularity rankings in 1895 to 874 a century later, and will most likely never gain traction with men again. Dana, Carol and Shannon met similar ends.

By contrast, Jordan has appeared in the Top 100 most popular names for both sexes since 1989, and other modern unisex names coexist peacefully, too. Angel, overwhelmingly male until the mid-’50s, became popular for girls around 1972. Yet boy Angels surpassed girls in 1986, and the name now sits at No. 31 for men, 160 for women. And the popularity of Logan for boys (it perennially appears in the Top 50) may have eroded its cachet for girls, an unusual reversal.

The best example of a new gender-fluid name is Peyton, which wasn’t popular until the quarterback Peyton Manning emerged. It tested parents’ tolerance of ambiguity, since it lacked strong gender connotations. The name caught on with girls first in 2000. But parents, perhaps hopeful for their sons’ athletic futures, loved it for boys too. Strikingly, its popularity with both sexes surged and dipped in lock step over the past decade — meaning parents responded to the fickle laws that govern name popularity identically, as if sex made no difference. Peytons of both sexes probably gained thousands of peers when Manning’s Colts won the Super Bowl in February.