I think that this is a change that is due to modern diapers. IIRC Dr Spock, writing in the 40's, advised that if they are left to their own devices most childre

I think that this is a change that is due to modern diapers. IIRC Dr Spock, writing in the 40's, advised that if they are left to their own devices most children will potty train themselves by age 2