With spring arriving finally, if belatedly, it’s getting harder to blame an icy winter for sluggish home sales. New York Times columnist Neil Irwin has another, more disturbing explanation — the difference between demand (what people want) and effective demand (what they can afford.) According to Irwin, many of the consumers who want to be homeowners can’t afford the cost, and so aren’t buying in large enough numbers to keep the housing market humming as an engine of the economic recovery.