Helen Fisher is an anthropologist who studies love and sex and their effects on the brain. Here are some thoughts from her on the changing dynamics of our family relationships. This is from an exchange between her and Krista Tippett on the On Being podcast

We’re seeing a new form called — that I call the association. And, I’m really excited about it because it’s groups of friends. [I]t’s an association of friends that is my real family. And it’s interesting how a lot of young people — they’re much closer to their association than they are to their own family. So Christmas and holidays become very stressful for them. Because they go home to families that they really don’t know very well, and who don’t really know them. They don’t know these people they way they know the people they hang around with in New York City. So we’re building new forms of family.

And this…

MS. TIPPETT: Well, and the other thing I’ve thought about some over the years is how marriages are such lonely — the nuclear family is very unnatural in human history for these same reasons, right? That marriages and families would have been embedded in networks of other marriages and other families and elders and cross-generational.

MS. FISHER: So well said. So well said.

MS. TIPPETT: And I think it’s like this death blow to marriage as an institution almost to have it be this isolated where you have two people who are left to take everything out on themselves.

MS. FISHER: People are so upset about this — a single mother, or a single father. I’m upset, like you are, about the two of them. They’re all by themselves.

Here’s the entire conversation: THIS IS YOUR BRAIN ON SEX.