At Nameberry, Pamela Redmond Satran has commented on a sign that there is more upcoming "gender equality" in naming: a popular girl's name inspiring the use of its masculine form(s), such as the example she gave of Emma for girls lending the way to Emmett for boys. She also commented on how she thought a generation or two ago that a boy's name being similar to a girl's name would be a minus for its usage on the boy's side. Actually, as I will demonstrate in this blog, that has not always been true. Indeed, many of the classic "unisex nicknames" get started first by the feminine form(s) being popular, and then following along or trailing are the masculine form(s).
The first one I'll take a look at is one that is still fairly popular and fashionable: Sam (Samantha, Samuel). Samantha's entrance into the mainstream name pool can be pinpointed to the TV series Bewitched, which premiered in 1964. Over the next quarter-century the name climbed the charts, and ended up being the fourth most popular name for girls in the 1990s. Although now given to less than a third of the number of babies as at its peak and slowly falling in popularity, Samantha still ranked at #17 in 2011 (which not qualifying as a true classic can certainly qualify as a "modern classic"). The masculine for, Samuel, does qualify as a true classic though (never being out of the Top 100 and a popularity spread ratio of less than 1-4 from its least to most popular years). Samuel's re-ascent in popularity happened almost right along with Samantha's (although less steeply), which goes to show that parents debating on Samuel for a boy did not let the prospect of also knowing girl Sams deter them. In fact, Samuel's peak was after Samantha's and while the feminine form is dropping the masculine form has remained fairly steady.
The next one is a name prefix that many parents-to-be grew up with a lot of: Chris (Christine/Christina/etc., Christopher/Christian/etc.). Christine was the leader of the pack, peaking in the 1960s. Christina and Christopher were the 70s/80s hits (thus being another case where the feminine forms led before the masculine ones). The Chris- names are largely in fashion limbo for girls (with all forms now well out of the Top 100), but Christopher for boys dropped more slowly (still ranked at #21 last year) and the other common masculine form, Christian, saw its heyday during the 90s and 2000's decade (starting to fall but came in at #30 in 2011). Although probably not the best example of feminine-to-masculine inspiration, it's still another example that boys and girls have not minded sharing nicknames even in the past.
The last one I'm examining is Pat (Patricia, Patrick). Patricia saw a huge rise from being semi-obscure at the start of the SSA list to one of the most popular names at the time of the post-war baby boom, and afterwards slowly but surely fell to become uncommon again among modern baby girls. It took a bit longer for the masculine form, Patrick, to see its peak (which was nowhere near the feminine form's) in the 1960s and remained fairly steady through the 1980s. This makes Patricia-Patrick a good example of feminine-to-masculine inspiration (and a case where the females that a typical boy Pat shared a nickname with were not his female classmates but his friend's mothers, teachers, aunts, etc., also considering that a Generation X/Y Patricia would be more likely to use one of the "back-end" Tricia-type nicknames than one of the Pat-type ones). (This might be good news for unisex names like Kelly, Robin, and Shannon now that they sound dated for girls and thus a modern boy with a name like one of those would have a low chance of sharing it with a female classmate, although he may well share it with an adult woman that he knows.)
(I apologize to Pam if she doesn't like that I used a tool from a competing name site to show the stats, but since that tool makes it easy to make the cross-gender comparisons on a single graph like I did I thought it was the best equipped for this job. I use ideas from both parties' name sites to build upon my ideas.)
Addendum: Although this one isn't nickname-based like the others, I thought of another (currently in-style) name in which the masculine form is climbing in the shadow of the feminine one (Olivia, Oliver). Olivia's been near the top of the charts for several years now, while (in the U.S.) Oliver is just starting to catch up (although at a pretty fast rate).