Nerd, as a stereotypical or archetypal designation, refers to somebody who pursues intellectual interests at the expense of skills that are useful in a social setting, such as communication, fashion, or physical fitness.

I was just reading an EE Times Article (See What drives you? Pick your brain), and my conclusion was that research indicated in the article can be applied to explain/understand Nerd(-iness).

Basic observation, made in the research is that brains have a systemizing mechanism which allows brain to digest predictable (systemic) information in rather simple manner. However, as the systemizing levels increase, the individual's ability to cope with spontaneous change severely decreases. Social settings like "the way people move around at a cocktail party" is entirely unpredictable and hyper-systemic brains are at loss in handling such situations.

Nerds, as popularly known, are good at intellectual pursuits and feel awkward at social events. See the connection!

Though experience (and I believe) have shown that this may not be the only reason. There is one interesting read @ Why Nerds are Unpopular by Paul Graham in this regards. Argument, put forward in this article is

Nerds aren't the only losers in the popularity rat race. Nerds are unpopular because they're distracted OR smart kids are unpopular because they don't waste their time on the dumb stuff you need to do to be popular.

I kind of agree with above argument!

Bottom Line: Our genes may define the directions for our journey but we may still choose to take detour on the road!