[quote Faultymain5]@SchadenfreudePersonified re research, do they compare those children who got baby talk as opposed to those who didn’t? And how do they make sure parents don’t fall into baby speak as it’s so instinctive. How is it all measured? I’m really curious as no-one has ever mentioned it to me before this post.[/quote]
I'm sorry - I can't remember any details - but it's definitely out there somewhere.
As other posters have pointed out - the high voice, sing-song intonation etc is attention grabbing for an infant, and the fact that (most!) mothers instinctively use this speech pattern suggests that it is evolutionary. I'll see if I still have anything related to it if I get time tomorrow (I've been retired for 10+ years and gave most of my books etc to a neighbour's DD who started training.
Simplified sentences are obviously better for children, too - but that doesn't mean that you have to use baby talk. If you aren't comfortable, you can say "dog" or "doggy" rather than "bow-wow", but the high, sing-song voice definitely engages the attention of infants.
There is a LOT of research describing the importance of smiling and eye contact. Children who are physically cared for, but not interacted with are much more likely to develop serious social and mental health problems. They also don't sensitively parent their own children when they have them - this is learned behaviour. (There have been a number of longitudinal studies - not deliberately depriving children long term of interactive parenting, obviously, but using information gleaned from the children of mothers (in particular - usually the primary caregivers) who are depressed and have a flat "affect" (manner), and children who have been brought up in the old care system.
(It's actually very difficult to get mothers and infants to take part in experiments like this, because the obvious distress of children who aren't interacted with upsets mothers too much, and they break character. They cannot not interact with their baby. It's almost always too upsetting for both the mother and the child.)
The horrors of the Rumanian orphanages also provided huge amounts of material for psychological studies.