I'm going with the Sterile Home hypothesis - overuse of bleach and other "disinfectants" in 1st world homes deprives kids of exposure to "germs". Those otherwise non-pathogens have some positive effect on the immune system such that we have an increasing incidence of allergies + asthma.
Allowing young kids to play outside and get dirty is going to be a tough sell to germaphobic helicopter moms who also can now fear CPS will arrest them for letting kids play outside.
