Anyway back to the names thing (although it looks like there should be a proper thread/bookclub on the nature/nurture discussion...)
There is a difference between the speed at which girls and boys names go in and out of fashion as thee marvellous graph shows. And has got to be something that is pure nurture, so it is interesting for what it shows about the different hopes and fears parents have for their girls and boys at the outset.
Exoticfruits - it doesn't make sense to say that John, Joseph, William etc.. have been so popular for so long because there are 'fewer boys names to choose from' as if that is a fact of nature. The reason there are fewer boys names is because people are more willing to use 'exotic' (e.g. French, Sanscrit), made up or fanciful names for girls. People are not more conservative in choosing boys names because there are fewe to choose from, there are fewer to choose from because people are more conservative with boys names.
My guess it is because there is a tendency for names associated with older women (Brenda, Linda, Doreen etc..) quickly loose their cachet, while names associated with older men not so much. I think people name their boys half thinking m is this the name that would suit a CEO, Prime Minister, boss etc..'. and not to want to give them a name that looks 'lightweight'. I could be completely wrong in this explanation, but the pattern that boys name and girls name fashions change quite differently look like it's real from the data, and I think that's interesting.