We in Britain and USA share much common heritage and included are our given names (what were called Christian names when I were a lad).

In Britain these tend to be pretty much traditional with a few exceptions. In the USA there are also many traditional names BUT, here's the thing, there is a preponderance of surnames (family names) used as given names. So we might have David, Michael, Susan, Mary alongside Washington, Jackson, Taylor and Macauley. As we know surnames came into being to distinguish people of the same (given) name once populations began to increase. (David son of John would be called David Johnson not just plain Johnson)

My question to our American friends is how did this about-face naming start?

BTW I once worked with a Harrington James and I always wanted to call him James Harrington (we settled for Harry)

As a subsidiary question - why are so many American girls being given neutral or male names?