There has been an undeniable shift in the conventional nuclear family over the last several decades, with increases in single parent, step-parent, grandparent and single-sex parent homes. A recent survey by the New York Times stated that as of 2009, women who gave birth under the age of 30 were for the first time less likely to be married than not. These trends, along with the rise in divorce rates seen over the last 40 years, support the relationship narrative put forward by Dr. Christopher Ryan and Dr. Cacilda Jetha in their book Sex at Dawn of long-term, semi-monogamous mates, rather than story-book life-long, monogamous marriages.
Ryan and Jetha question the naturalness of our society's emphasis on exclusive and eternal pair-bonds. They cite the polyamorous relationship tendencies of our early ancestors and the sexual habits of our closest living relatives, free-loving bonobos and chimpanzees, as evidence that we were not always this way. They also list current hunter-gatherer societies who maintain looser definitions of sexual relations and expectations, and who have adjusted their property owning and child-rearing practices accordingly. Examples include the Curripaco tribe of Brazil, where a couple who hang their hammocks together are considered married (until one hangs her hammock elsewhere), or the Dagara of Burkina Faso, one of many societies in which every woman is called "mother" and every man "father" as questions of paternity are highly uncertain. Allocations of resources and affection are spread equally throughout these tribes, treating all as family and ensuring the well-being of every member of the community.