Your family has a huge influence on your politics — whether you like it or not. It’s well established that the political values of parents hugely influences their children’s party affiliation. Now, a new study reveals that family influence goes beyond if your parents lean blue or red: It’s the size of a family — how many kids there are, as well as how many siblings you grow up with — that is positively correlated with how conservative your family values trend.

“The study makes essentially a basic point, which is that family size and social attitudes are correlated,” study co-author Tom Vogl, a development economist at the University of California, San Diego, tells Inverse. It also demonstrates, he says, “the consequences of that correlation for the composition of the population and how it affects public opinion, population-wide”.

The findings were published Monday in the journal Proceedings of the National Academy of Sciences.

Political polarization is a defining feature of American politics — and polls indicate that negative feelings directed at opposing parties have increased in recent years. Despite this, the trend towards liberalism has been slowly but steadily gaining traction over time, with the American political mood now thought to be the most liberal it has ever been.

However, the findings of this new study suggest that a specific form of conservatism — one that is interested in family values, called “traditional-family conservatism” — still persists generation after generation, despite generally increasing liberalism. The bigger your family , the more likely you are to uphold these values , the research suggests.

How the size of a family influences politics

Vogl teamed up with Jeremy Freese, a sociologist at Harvard University, and together they analyzed the responses of 12,017 adults, all over 25 years of age, who participated in the US General Social Survey between the years 2004 and 2018. Specifically, they looked at the views of the respondents towards two contentious issues: abortion and same-sex marriage.

They proposed a fake scenario: Instead of families of all different sizes, what if all the parents in the previous generation had only one child — and what if that one child wound up having exactly the same attitudes as they do in the real world today, but they were representing the population as if all of their parents had exactly one child?

When they applied that scenario to the data, the team found that the likelihood that survey respondents had opposing views towards same-sex marriage and abortion rose by between 3 and 4 percentage points than if traditional-family conservatism had been independent of family size in the current generation.

"Natural selection is a silent warrior in America’s culture wars."

More siblings linked to a rise in opposition to abortion: When sibling number was accounted for, opposition spiked from 53 percent to 57 percent. In the case of same-sex marriage, disapproval rates increased from 38 percent to 41 percent when family size was plugged into the equation.

Vogl explains that something called “differential fertility” is thought to be the underlying root cause of this rise in conservatism – a term which, Vogt explains, just means that “across families – at any given point in time – the characteristics of those families are associated with how many children they have.”

“These attitudes are correlated with fertility; with how many children people have,” Vogl explains. “Parents and their children tend to share similar attitudes, and that induces a relationship between childhood family size and attitudes in adulthood.”

This pattern may have an evolutionary explanation .

“We have this theory that this relationship exists because cultures, societies, religions were all interested in promoting their demographic power,” Vogl says. “This suite of ideas about the family – about women's role in the household, about whether homosexuality should be prohibited, about whether abortion should be prohibited – all sort of coalesce around promoting larger family sizes.”

He refers to these values — such as opposing abortion or same-sex marriage — as being “pronatalist”; basically meaning that they increase family size, by encouraging reproduction.

In turn, the team writes that opposition to the two issues that have defined the “culture wars” that have shaped American politics for the last 50 years — abortion and same-sex marriage — would be significantly less prevalent if family size didn’t have this influence.

“Natural selection is a silent warrior in America’s culture wars,” they write. “These forces can help sustain large pockets of opposition to change, even in the face of broader liberalizing trends.”

Abstract: Data from the General Social Survey indicate that higher-fertility individuals and their children are more conservative on “family values” issues, especially regarding abortion and same-sex marriage. This pattern implies that differential fertility has increased and will continue to increase public support for conservative policies on these issues. The association of family size with conservatism is specific to traditional-family issues and can be attributed in large part to the greater religiosity and lower educational attainment of individuals from larger families. Over the 2004 to 2018 period, opposition to same-sex marriage and abortion was 3 to 4 percentage points more prevalent than it would have been were traditional-family conservatism independent of family size in the current generation. For same-sex marriage, evolutionary forces have grown in relative importance as society as a whole has liberalized. As of 2018, differential fertility raised the number of US adults opposed to same-sex marriage by 17%, from 46.9 million to 54.8 million.