Theo has a girls' name does not even make the five-name-cutoff on the Social Security list today -- there were fewer than five girls (and possibly none!) named Theo in 2012. Not so in the half century from 1880 till 1930, when Theo was in the Top 1000 for girls. Nicknames given as full names were stylish then, as were boyish nicknames such as Billie and Tommie for girls. Theo fits this old-fashioned trend, but these days, as Theo climbs assertively back onto the Top 1000 for boys, we see it as a male name.