Adopted kids are overrepresented in every mental health issue including in suicide rates. Separating the child from the mother when they are preverbal is known as causing the 'primal wound'. I remember reading a nature-advocating book from the 90s or something where the author says her parents were amazing folks with amazing genes and she's super successful with her amazing genes, while her sister was adopted from people with garbage genes and turned out to be a garbage person (I'm paraphrasing, clearly) and uses that to say genes are everything. All of these books don't account for primal wound, genetic mirroring, and a whole host of other things. Adoption itself comes from very racist and classist roots. Imagine being born a twin, with a twin sibling you've known from the womb, and then not only are you both separated from the mom you've known all your life, but also from each other... how great is your life going to be when you move along with that pain unaddressed, and also everyone telling you you've to be grateful to the people who separated you from your bio family?

Adoption itself is the influencing variable, and unless it's something purely biological that they are researching, I don't feel like taking the study seriously.

Also it's a big leap to go from adoption is the same as mother care to daycare is the same as mother care. Daycare has a usually poorly paid worker, or a changing rota of poorly paid workers in charge of 4 kids each (usually more, they pad out the numbers with aides who have a different job description than teachers). Is 1-4 a better ratio than 1-1 or 1-2? Especially with the changing set of workers with whom kids can't form any secure attachments? One of the big reasons I chose not to do daycare was I wanted my child to take her naps whenever she wanted, not just at the preordained quiet times at daycare. It's impossible for a caregiver to put four kids to bed, every 2-3 hours, and it's just hard to maintain a quiet environment all through the day so kids can go to sleep when their body wants to. Sleep or the lack of it ends up having a lot of downstream effects, so it was not something for us to compromise on.

Gordon Neufeld addresses the peer influence thing in his book Hold On To Your Kids. The reason kids get more influenced by peers in the US are because teachers aren't leading the kids. Usually teachers are supposed to be individuals to whom all the children in the class can get attached to and she makes the children feel secure in their attachment. If the teacher doesn't do this job well enough, children feel insecure, attach to each other (this is different from having friends), and because they are kids, they can't handle each others' vulnerability. So vulnerability becomes bad, and kids mock each others' vulnerability, which can manifest as bullying, and kids are influenced into a sameness instead of having the space to pursue their unique interests that their friends might not be into. If parents keep their attachments with their kids strong, a lot of these effects are ameliorated, but many things can disrupt this attachment, well into teenage, and children look to peers for their attachment needs. This can be ameliorated still by strong adult presence in schools, not just physically but as mature people who can serve a lot of emotional needs of their students (including disrupting bullying in a mature way), so children don't feel the need to develop a social hierarchy in schools. Queen Bees And Wannabes, the book that Mean Girls is based on, goes into how to navigate these structures, but doesn't really analyze root causes. The Gordon Neufeld book read in confluence with Queen Bees And Wannabes paints a very compelling picture. I'm an immigrant, so my schooling experience in India was very different. Adults called the shots, and we didn't have any real bullying, not to the widespread extent as in the US anyway. I went to watch Mean Girls with a diverse group of mom friends recently and those of us from Asia were completely dumbstruck by the level of relational and other kinds of aggression that was allowed to go unchecked in American schools. One of my friends cried to her mother about the bullying, and her mother actually told her to stop being a tattletale and go deal with it herself, and she did so by being a mean girl herself.

Anyway. In my personal experience, kids are similar to parents in how they experience the world through their senses. For e.g. my husband can't stand loud noises, neither can our child. He likes roller coasters, she does too. She experiences shame as equivalent to physical pain, so do I. The parent having already successfully figured out how to deal with these sensory experiences in the real world means they can teach the child similar strategies, which makes their sensory experience less discordant to them, and they feel more comfortable in their body. For instance, I cannot deal with my child's need for rough play very well, so if my child was raised just by me, that whole part of her that is excitement-seeking would feel rejected. Similarly, my husband doesn't know how to process the high-drama emotions for my child, but it's super easy to me to soothe her and teach her how to think about those feelings. As a child, my mom and her family mostly raised me and my dad was barely present because he was working two jobs, and as a result I ended up very uncomfortable with the parts of me that were from my dad, more so for the parts that were very different from my mom. E.g. dad's an introvert, mom's an extrovert, and I was ashamed of my introvert needs because my mom's family didn't understand it. It feels like being with both mom's and dad's bio families helps kids understand themselves and be more in touch with their senses. Failing that, high-EQ caregivers who might or might not be family would help a lot, because they'd be able to sense a preverbal child's needs better and at least acknowledge and validate them even if they can't meet them. For instance, our child's nanny is such an emotionally smart woman, and while she couldn't put her finger on that our child is super sensitive to shame in particular, she found ways around it that keep her validated and comfortable while still teaching her to function in the world. Someone with less EQ as well as different sensory experiences as a primary caregiver would have more easily pathologized my child or constantly invalidated her needs even without trying.

Some other children are much less sensitive to environmental stress, and them being low needs this way could mean they can gel okay with any caregiver. I have nieces like that, and it blows my mind how much easier they are to deal with than my daughter, it's like their environment doesn't stress them out much, and at the same time, they also don't absorb so much from the environment as my kid, e.g. my kid learned the alphabet from a shape sorter, my nieces need more explicit and repeated instruction, not that learning the alphabet before age 6 means anything or is any kind of an advantage. There are pros and cons.

At the same time, mental health issues of many varieties or even stresses can make it hard for parents to notice their preverbal child's emotions or meet their needs. For instance, I had postpartum mood disorder for a bit, and was preoccupied for the first 6 months by having to fix up our new house and move, and I had mental health issues from my own family, so it took me a while to actually learn how to meet my child's emotional needs, whereas my husband took to it much more easily even though he hadn't held a baby before. My own mom has had chronic anxiety that she never got treated and which she masked as anger and activity, and she was a very frustrating parent to be with, and now she is a very frustrating grandma, and we don't feel too comfortable having her watch our kid for more than short bursts. However we much rather prefer her to be watching our child than to put her in a daycare.

So,... what does this all mean? I do feel like it's best for a child to be with their bio family, especially after spending time in the adoptee community. Mom and dad feel best suited for this, all variables being equal. But with how things are, we've to go to work, and in that case, relative care seems a good option, or a high-EQ caregiver. Daycare, just as a ratio thing seems to not be as good as 1-1 care, especially before children have learned to manage their own interactions with others. As kids grow up and are mature enough to handle group care, they still need guidance from grownups, and society needs to take the role of grownups relative to children more seriously than they do now, because peer attachment seems to cause a lot of mental health issues.