That idea came about in the 60s when some famous baby doctor wrote some idiot book about babies and claimed putting babies on their backs prevented SIDS. He made these claims with no scientific evidence, btw, but the mom's just ate his book up. When the scientific community went "hmm this...doesn't seem to be working", they did an actual study on it and found that making babies sleep on their back was actually DETRIMENTAL.

Apparently there was a massive PR campaign in the UK when this information came out and within a year or so of parents letting their babies sleep on their stomachs, SIDS rates had plumetted.


((We talked about this case in our Epidemeology/Evidence-Based Medicine course, so I don't have all the details.))