One of my favorite blog finds of late is Strange Maps. From that source, comes this map guaranteed to bring fear to the heart of every thirty-something Manhattan single woman:
My guess is that a subtitle to the graph in southwestern cities is "single men immigrate more than single women." Note the scale really under-estimates the situation in NY, since the scale is capped at 40,000. If they had extended the scale, NY would be probably be a 2-inch diameter circle.