Obvs a study based on 25 babies isn't useful.
I loathe the idea of leaving babies to scream themselves to sleep. I have a poor sleeper, now a toddler and the pressure put on me to leave him to scream has been huge - from health visitors, from other mothers. Along with an often sneery put down about how I will 'spoil' him if I co-sleep, or some odd comment about how I must enjoy being a martyr if I didn't do some form of 'controlled' crying.
I hate how CC mothers often talk about their baby as if they have no emotional needs. "he's been fed, his nappy's fine, he's warm enough - there can't be anything else wrong with him.' I also hate how so much comment about behaviour is based on twaddle cod psychology. Like if a baby is cuddled when they are awake and screaming at 5 months old, they won't be able to cope with not having their every whim met as a four year old at school. Self serving twaddle.
As a cautionary tale - turns out my ds had reflux, and being laid down on his back was very painful for him, as acid digestive fluids rushed up his throat when he was in that position. we didn't realise for ages - new parents, knew no better, and it was only after realising he was actually gagging and screaming when he was put down that we put two and two together. Even with a propped up bedside cot it was an issue - one night I woke to hear him cough and choke, all of a sudden his legs were thrashing and he just wasn't breathing. I grabbed him, got him upright, and he choked and coughed and was sick everywhere. He was very badly distressed and panicky for ages afterwards.
I firmly believe if that had happened while he'd been left alone to scream, he would have choked and died. He couldn't even roll himself over at that age. Checking him twenty minutes after he'd gone quiet would have done fuck all.