Apes and Humans - Cognitive Differences:

[Although] apes and humans share a number of important advanced cognitive abilities, they differ in one key respect: the extent to which humans can detach themselves from the world as they experience it. This allows humans to reflect on the world as they find it, to wonder wether it could have been otherwise. In contrast, apes (and certainly all other animals) have a much more direct, straightforward experience of the world. Their noses are thrust firmly up against reality.

The Human Story, Robin Dunbar

Kith and Kin:

"While you may have no choice about the family you have, you can at least choose your friends. We and others have found that friendships tend to be characterised by similarity in likes and dislikes, known technically as homophily (love of similarity). It seems that the more things we have in common with someone, the closer the friendship will be. In our study, we identified  five key traits that seemed to be particularly potent in creating friendships: having the same sense of humour, the same hobbies/interests, and the same moral values, having a similar level of education/intelligence, and having being born in (or, at least grown up in) the same area. The more of these five traits you share with someone, the greater is your emotional closeness to them, and the more likely you are to help them out in time of need."

Robin Dunbar, The Science of Love and Betrayal

Monogamy and Brain Size:

Among the birds and mammals in general, the spezies with the biggest brains relative to body size are precisely those that mate monogamously. Those that live in large anonymous flocks or herds and mate promiscuously have much smaller brains.

The birds make it especially clear that the real issue is strong, resilient, long-lasting pairbonds. Birds that mate monogamously come in two quite different kinds. There are those, like many common garden birds such as robins and tits, that chose a new mate each breeding season. But there are many others, such as many birds of prey, the owls and most of the crow and parrot families, that mate for life. It is this second group, which have the biggest brains of all among the birds, far bigger than those that are seasonally monogamous, and this is true even when we control for differences in lifestyle, diet and body size.

Robin Dunbar, How many... Dunbar's Number and other Evolutionary Quirks