Summary: There are many studies done, but they all have data quality issues to different degrees. In general... (my conclusion) > Leaving your baby to cry it out is helpful for most babies. > But it's not recommended for babies younger than 6 months. > Some experts recommend even waiting until 12 months, because month 6-12 is critical for developing emotional regulation that occur with help of parent intervention. >…
I'd add: > When babies are sleep-trained (with the "cry it out" method), they don't actually sleep (much) longer; they wake up as often but have learned to not signal their parents.
To instinctively avoid signaling the predators in the absence of their parents as an evolutionary trait...