Daniel P. Aldrich, an associate professor of public policy at Purdue University, is the author of “Building Resilience: Social Capital in Post-Disaster Recovery.”

HURRICANE Isaac, which made landfall in Louisiana last night, has not only disrupted the Republican National Convention but also brought back painful memories of Hurricane Katrina, which devastated the Gulf Coast seven years ago this week.

In August 2005, my wife and our small children and I evacuated to Houston just before the storm destroyed the New Orleans home we had moved into six weeks earlier. We took with us just a bag of toys and a suitcase. We applied for federal aid, but especially in the immediate aftermath, it was family, friends and friends-of-friends who came through for us.

As a political scientist (I taught at Tulane at the time), I decided to study how communities respond to natural disasters. I’ve concluded that the density and strength of social networks are the most important variables — not wealth, education or culture — in determining their resilience in the face of catastrophe....