Sally Ritchie is the youngest of eight children, but when it came time with her husband to start their own family, she had no illusions about repeating the pattern of the stereotypical big happy family.

"It was either one or two, but after I had one, that was that,'' says Ritchie, whose son Graham is nine. "Nowadays, it's very, very difficult to have more than one child and be sure that you're going to be able to put them through university and provide them with the home you want to provide them with.''

"And, frankly, you want to do better for your kids than was done for you . . . and I couldn't afford to do that if we continued with growing the family,'' she says. "And it's vitally important to me that I have a career.''

The Toronto couple's decision to stop after one child is hardly unusual in Canada. Statistics Canada's latest census figures released Tuesday show a continuing trend towards smaller families, with the country's fertility rate stalled at 1.5, the same as five years earlier.

The fertility rate -- the average number of children a woman will have between age 15 and 49 -- is far below the 2.1 per woman it takes to replace the dying population. Even so, Canada still beats out many other developed countries in the baby sweepstakes. Japan's 2006 rate is estimated at 1.4; Russia's at 1.39 and Spain 1.28.

But what's particularly striking about the 2006 census figures is how many fewer babies Canadian women are having compared to their neighbours in the United States.

In the postwar baby boom years, Canadians were pumping out more children than Americans, a situation that held until the late 1960s, when the Vietnam War caused a "little spike in fertility because if you were a new young daddy the draft board might not send you to Vietnam,'' says David Foot, a University of Toronto economist.

The U.S. rate stayed below Canada's until the 1980s, but since then there has been a significant divergence, with American women now averaging two children each, compared to 1.5 on this side of the border.

Theories abound.

In Canada, better contraception, career women who delay marriage and babies as they establish themselves in the workplace, and a divorce rate of almost 50 per cent are keeping the birth rate down.

In the U.S., the large Hispanic population -- accounting for about one in seven Americans -- is pushing the national birth rate up. Hispanics have about 2.9 children per family.

The closest high-fertility equivalent in Canada is First Nations peoples. But that group accounts for less than five per cent of the population, and so the higher aboriginal birth rate has little effect on the national average.

The teen birth rate south of the border is twice what it is in Canada. The number of teenage girls having babies is especially pronounced among America's black population, which often has fewer opportunities in education and employment.

"Fertility is primarily driven by female education,'' says Foot. "If you're poor and not in school you're having kids.''

As well, studies show that a higher proportion of Canadian women use more effective contraception than American women. That -- along with a lower average age for marriage -- could also contribute to higher U.S. birth rates, says sociologist Robert Glossop, a senior research associate at the Vanier Institute of the Family in Ottawa.

"In the States, for whatever reasons, you have more people who marry younger and therefore are more likely to have fulfilled their fertility aspirations by the time their reproductive career comes to an end,'' Glossop says.

Phil Morgan, a sociologist at Duke University in Durham, N.C., agrees that the Hispanic and teen fertility rate do pull up the U.S. average, but they alone don't account for the difference.

"When people try to get around high U.S. fertility with those kinds of explanations, I think they're missing it,'' Morgan says. "Because if you took white, college-educated women in the U.S., they have a fertility rate that's . . . about 1.65, 1.7 kids.''

"There's something about the U.S. that's different.''

Many experts believe that what sets the U.S. apart from Canada and other developed countries is its level of "religiosity.''

Loosely defined, religiosity is a propensity to have faith in God and to attend a house of worship. But it also tends to include sticking to a traditional family structure, with men as the breadwinning head of household and women primarily as nurturers of children.

"People suspect that this issue of the U.S. being more religious than Canada may have something to do with the differences in fertility rate,'' says Prof. Nathan Lauster of the Social Work and Family Studies program at the University of British Columbia.

"There are a variety of reasons why it might have an impact on fertility,'' says Lauster. "Some of that may be more cultural than anything else, it might be something about how people think they're supposed to act in life, which is influenced by their religious upbringing or by the group of people that they get their ideas from about what family life should be like.''

Morgan of Duke University is about to publish a study on religiosity, in which Americans were asked a number of questions, including the value they place on religion -- from very important to their lives, to somewhat important, to not at all.

"The difference between not at all and very important is a whole kid,'' he said of the correlation between religiosity and birth rates. "So it's huge.''

Amelie Quesnel-Vallee, a social demographer at McGill University in Montreal, says American women may be having more children than other women across the globe, but that doesn't mean a better life.

"Definitely they're having more children, but they're not necessarily giving them the same life conditions that Canadians would,'' she says, pointing to the proportion of U.S. children living in poverty.

While income disparity among Americans is hardly ideal, Morgan believes it ends up favouring a higher birth rate. "The inequality in the U.S., which is pretty extreme, means there are lots of poor women around to look after rich women's kids.''

As well, the massive, 24/7 U.S. economy means many couples can work opposite shifts, allowing them to bring in a double income and still raise children.

Quesnel-Vallee believes Americans also tend to more optimistic than Canadians.

"There really is this sense in the face of everything that's contrary that it's the land of opportunity. And so there is that playing into it, where individuals are just having children no matter what.''

A sub-replacement birth rate can have far-reaching implications for Canada. There will be fewer young people in the labour force to provide pension- and health-care support for the large bubble of baby boomers approaching old age. And so Canada will need to rely more and more on immigration to bolster the population.

Quesnel-Vallee believes Canada has probably hit the bottom of the birth rate trough and that the "pendulum of human behaviour'' will begin swinging the other way.

"Maybe it takes a shot to go down low and be concerned that we won't reproduce ourselves as a society to then go back up.''