[quote ThatsNotMyCherry]Thanks! This seems like an interesting read on this subject. Just discovered it after posting this thread.
www.goodreads.com/book/show/270100.Mother_Nature#[/quote]
Sarah Hrdy is brilliant and I enjoyed both Mother Nature and Mothers and Others.
My impression (based on both personal experience and my amateur readings of psychology) is that babies exist in a non-verbal world in which physical bonding is important. The biological mother has an advantage in bonding because of the physical acts of pregnancy, childbirth, and (optionally) breastfeeding, but a father or adoptive mother can also bond through consistent physical contact, while other adults like grandparents and childminders can provide supplementary bonding (as well as support for the mother, which is also important in maintaining her mental and physical health).
As a child becomes more verbal and mobile, their social needs become less intensely physical and branches out to include other adults and children. The unit of childrearing becomes less nuclear family and more "the tribe".
I'd be interested in other studies and viewpoints.