I thought the original attachment theory of the 1950 never mentioned mothers - just one care giver could be any adult - but others interpreted this as the mother.
I also thought most of it was a reaction to the very prescriptive child rearing that went before 4-hour feeding, leaving to cry and putting at bottom of garden for naps.
The working class women had to work whether that be taking in washing, cleaning etc. The children were often there but mum was not focusing on them singing ‘the wheels on the bus’. A lot of the time the kids would be left with another bunch of kids of varying ages, an older child, a grandparent, unrelated woman or just left to their own devices for huge swathes of time
That pretty much how my parents were brought up – my MIL found cleaning work she could take DH to as a young child. My parents managed as Dad’s work paid overtime – wouldn’t now and then Mum found work around school later on. One of my regular delivery people was a mother with her toddler in her small van – she only did a few hours’ couple of days a week.
I also disagree that children not in nursery don’t socialise– I was home with mine in two different locations and there were plenty of toddler groups and classes – though not all people there were SAHM – many worked p/t or other relatives took the children grandmothers, aunts and occasionally Dads and sometime child minders. I also wasn’t only parent attending these groups with more than one child.
I’m not sure how outcomes would be measured – a large number of parents I know use a mix of childcare such as nursery, child minders, family and pt or condense working or working shift around each other but that never seems to get mentioned.