The map comes from Richard Florida, who explains the west coast surplus of men this way:
Each of these regions has grown substantially over the past two or three decades, offering jobs in everything from high tech to construction and services. As numerous studies of migration show, men — especially those in regions with declining economies — are initially more likely to move long distances for economic opportunity, while women are more likely to stay closer to home and family.
Uh huh. Consider me unconvinced. On the other hand, no persuasive explanation leaps to my mind either, so I guess I shouldn’t complain. Anyone want to take a whack at this?