How dirty should my kid be?

LEFT TO THEIR OWN DEVICES, most children won’t hesitate to, say, lick a doorknob or wipe snot with their sleeve. But is there any truth to the idea that their affinity for getting dirty can be beneficial to their health?

That theory dates to the 1800s, when European doctors realized that farmers suffered fewer allergies than city slickers. However, it didn’t gain widespread attention until 1989, when British epidemiologist David Strachan discovered that youngsters with older siblings were less susceptible than other kids to hay fever and eczema. Strachan suggested that…