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No two families are alike. There are big families and small families, blended families, childless families, and families that no longer have parents.

We all know this, even though there’s still a pervasive cultural norm that families aren’t “complete” until they have at least two children. But that concept of a nuclear family with multiple kids is starting to break down, as more and more couples opt to stop after one child.

Statistics Canada has found that the number of families with just one child is increasing slightly. Single-child homes made up 37.3 per cent of households with kids in 2001; in 2011, that number went up to 38.6 per cent. The total fertility rate that year was 1.61 children per woman, and by 2016 it had gotten even lower: 1.54 children per woman.

That’s “the lowest level observed since 2003,” and “close to the lowest total fertility rate observed in Canadian history,” Statistics Canada noted.

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This is on par with numbers from around the world. In the U.S., the average number of children has gone down from 2.5 children to 1.9. In England, 46 per cent of families have one child. In Spain and Portugal, it’s about 30 per cent, and it’s also becoming more common in Italy, France and Germany, The Toronto Star reports.

But still, there’s a stigma.

“If you have one child, you are made to feel guilty for not having another,” Susan Newman, the author of The Case for the Only Child, previously told Psychology Today. “Your mother, your friends, even strangers tell you that “You can’t have just one. How can you do that to your child?”

Many of the myths about only children — that they’re spoiled, badly-behaved, and aren’t fully socialized — persist, even though those finds mostly come from a single 1896 study that has since been debunked. More recent peer-reviewed studies find that only children aren’t actually all that different from kids with siblings, Time has reported.

(One exception: only children do score higher than average in measures of intelligence and achievement, something they share with firstborns and children with one sibling.)

Watch: Why only children may be more creative. Story continues after video.

There are several reasons why only child families are on the rise:

The economic cost of having a child

Kids are not cheap. According to MoneySense, the average cost of raising a child to age 18 in Canada is $253,946.97 — a number that increased by over $10,000 between 2011 and 2015. The single biggest cost is childcare, but food, clothing, health care, increased transit costs and recreation are all significant, too. When the New York Times polled Americans who didn’t want large families, the most common answer they received was that child care is too expensive.

FatCamera via Getty Images Group of kids sitting on the carpet in their kindergarten classroom.They are clapping along to music.

Young Canadians have more household debt and less secure jobs than their parents’ generation, and home ownership is out of reach for many of them. It stands to reason that many don’t have the financial means to raise large families.

The changing role of mothers

As women continue to become more active in the workforce, they’re starting families later. From the 1950s onward, the average age at which a woman had a first child was 24, but that number has risen steadily since the mid-70s, according to Statistics Canada. In 2011, the average age at first birth was 28.5 years old — the oldest age on record. The average age for births generally, in 2016, was 30.8.

For women in particular, having children can often still mean facing setbacks at work. An RBC report earlier this year found that mothers can face less pay and fewer work opportunities for up to five years after a baby is born. “Whether by design or circumstance, women may already be working to limit the hit to income by delaying having children,” the report says.

“There is no getting around the fact that the relationship between gender equality and fertility is very strong,” sociologist Philip Cohen told the New York Times. “There are no high-fertility countries that are gender equal.”

The inevitability of climate change

For some, that means keeping the number low.