Amy Blackstone is a sociology professor at the University of Maine.

“We got a puppy, and that’s my idea of starting a family. People say, ‘Oh, that’s practice for parenting,’ but if it’s practice for anything it’s to be a mom to another puppy.” –Christina Hendricks

Mad Men’s Christina Hendricks is the most recent among a host of celebrities to be asked about when she’ll be adding kids to her family. Though the media has only recently taken notice of the childfree, the fact is that rates of childbearing in the U.S. have been on the decline for the past 40 years. It seems celebrities aren’t the only ones choosing to create families that don’t include kids.

The notion that family is something we choose rather than something based solely on ties of “blood or marriage” isn’t new. Kath Weston explored this idea over two decades ago in her 1991 book on gay and lesbian kinship, Families We Choose. Yet Google “start a family” and you’ll quickly discover that for many people, even today, families don’t begin until children enter the picture.

In 1976, just 10 percent of women had not given birth by the time they reached their forties. Today, that number has nearly doubled, reaching 19 percent in 2012. While a fifth of women may be without children, they are not without families. Research shows that people without children form bonds, create households, and help rear the next generation in many of the same ways that those with kids do.

For the 45 childfree women and men I have interviewed in the course of my research on the choice not to parent, family is about belonging, social support, responsibility, and love. For my interview participants, family can and does include blood relations such as siblings and parents and it also includes partners with whom they may have legal ties. But, on the whole, their definitions of family emphasize the needs that families meet and the functions they fulfill rather than who their families do or do not include. As Sara, a partnered childfree woman in her mid 30’s put it, family is those who are “united despite any kind of differences; it’s a togetherness.”

Perhaps many of the definitions of family my research participants shared emphasize meanings rather than members because of childfree people’s own experiences of exclusion. A number of my interview participants shared stories about not being invited to events at friends’ and relatives’ houses because it was assumed, without asking, that they wouldn’t want to participate if kids were present. Others described how “family friendly” events in the community exclude their adults-only families.

Annette, a 40-year-old childfree woman who defines family as “anyone who cares for and loves each other” shared her frustration: “Our town has lots of great activities and most of them are called some variation of, ‘Family Fun Day.’ So does that exclude me? It usually does because it’s geared for children, not for my family.” It seems that family fun days and family friendly environments really mean fun and friendly for just one kind of family: those that include children.

Americans of course aren’t the only ones whose perceptions of family seem to be limited to household units that include children. In Ireland, couples without children are defined by the census as “pre-family.” In some ways, this makes sense; having children is an important milestone and children are an essential part of family for many. But when one fifth of women end their childbearing years without having had children, perhaps it is time to consider that not all families do, nor must they, include children.