Put babies to sleep on their stomachs

What actually happened

The prone recommendation was not folk wisdom, it came from pediatric authority, including Benjamin Spock's bestselling manual from the late 1950s. The reasoning sounded sensible, and it was never tested. Epidemiology in the late 1980s in the UK, New Zealand, and Australia found prone sleeping was one of the strongest modifiable risk factors for sudden infant death, and campaigns there produced immediate drops.

The AAP changed its recommendation in 1992, and the US Back to Sleep campaign launched in 1994. The US SIDS rate fell from about 1.2 per 1,000 live births to roughly 0.5, and comparable declines followed in every country that ran the campaign. It is one of the most successful public health interventions of the era, and it consisted of asking parents to turn babies over.

The choking fear turned out to be backwards: anatomy means that in a supine infant, the trachea sits above the esophagus, and reflux drains rather than pools. Prone sleeping is associated with rebreathing exhaled air, overheating, and impaired arousal. This is the sharpest example on this site of confident advice, given by experts, to millions of parents, that was wrong and lethal, and of what changing it accomplished.