How many variables can humans process?

In a new study, cognitive scientists show that humans can usually track just four mental variables when trying to solve a problem…. Link  [via]{style="FONT-STYLE: italic"} [Boing Boing]

I would definitely suspect that programmers are better than average at tracking variables but I wonder if we have a gift for this (the ability to track five or six or seven), or are just good at swapping to and from our four registers?