Left-leaning Americans are driving the U.S. birth decline, new study finds

A recent study published in Scientific Reports suggests that political beliefs are increasingly linked to the number of children Americans choose to have. The findings indicate that while conservative individuals tend to maintain birth rates near historical averages, left-leaning individuals are having significantly fewer children. This demographic trend provides evidence that differing birth rates are a main driver of recent fertility declines in the United States.

Political attitudes are often thought of as simple opinions. However, research suggests they are relatively stable traits shaped by both social upbringing and genetics. Because parents tend to pass their political orientations down to their children, differences in family size among political groups can influence the ideological balance of a population over time.

Martin Fieder and Susanne Huber wanted to understand how this relationship has evolved. “As evolutionary biologists, we are interested in why left- and right-wing political orientations did evolve and whether they are subject to natural selection through differences in fertility,” explained Fieder, an associate professor of evolutionary demography at the University of Vienna. “Earlier studies showed that political orientation is heritable and that fertility patterns differ across the political spectrum.”

Demographers study how societies transition to lower birth rates as they become wealthier. This shift involves cultural changes like a greater focus on individual achievement and non-traditional family structures. The authors aimed to explore whether this general decline in birth rates is happening evenly across the political spectrum.

“While globally both political extremes often have higher fertility, the pattern in the US and Europe is different,” Fieder said. “Individuals with more conservative political views tend to have more children than those on the political left.”

These new findings build upon previous work conducted by the researchers. In a 2018 study published in Frontiers in Psychology, the researchers analyzed global survey data. They found that in many regions, individuals at both the extreme left and the extreme right had higher fertility than political moderates.

However, their analysis of United States data in that same paper hinted that a unique advantage for conservatives began emerging during the 1990s. More recently, a 2024 study by the same authors in Biodemography and Social Biology indicated that European conservatives tend to have more children and grandchildren than their liberal counterparts.

“In a previous study, we demonstrated that these fertility differences are likely to contribute to a gradual demographic shift toward more conservative societies in Europe,” Fieder said. “In the present study, we wanted to determine when this divergence in fertility first emerged.”

To explore these trends, the scientists analyzed data from the United States General Social Survey collected between 1970 and 2022. The sample included 22,975 adults, consisting of 10,681 men and 12,294 women. All participants were over the age of 40, meaning they had generally completed their childbearing years.

The participants were grouped into 16 birth cohorts based on five-year intervals. This started with those born between 1903 and 1907 and ended with those born between 1978 and 1982. The researchers measured political orientation using a seven-point scale ranging from very left to very right, grouping these responses into three broader categories representing left, center, and right orientations.

To evaluate how fertility impacts the spread of political traits over time, the researchers used a statistical tool often applied in evolutionary biology to measure phenotypic selection. This method allows scientists to measure how strongly a specific observable trait influences reproductive success.

The data revealed a pronounced change in how political beliefs relate to family size. For individuals born in the early 1900s, political orientation had almost no association with the number of children they had. However, beginning with the cohort born between 1943 and 1947, a massive divergence emerged.

“We expected these results, but not to such a dramatic extent,” Fieder told PsyPost. From the mid-century cohorts onward, individuals with right-wing political views maintained birth rates at or slightly above the replacement level. The replacement level, typically considered to be 2.1 children per woman, is the rate needed for a population to replace itself from one generation to the next without immigration.

In contrast, the birth rates of left-wing individuals dropped sharply, falling well below the replacement level in the more recent cohorts. The authors noticed this drop aligns with historical changes in family planning. “We found that the gap began with the introduction of modern contraception,” Fieder said.

“Fertility declined markedly among individuals left of the political center, whereas those right of the center generally maintained fertility close to or above replacement level,” Fieder added. Individuals with centrist political views fell somewhere in the middle. Their fertility rates remained slightly below the replacement level but were consistently higher than those of left-leaning individuals.

“These findings provide new insight into the demographic drivers of fertility decline and suggest that differences in fertility may contribute to long-term changes in the political composition of the population, given that political orientation shows substantial parent-offspring similarity,” Fieder said. The research suggests “that much of the ‘rightward shift’ currently observed across many countries is related to underlying demographic changes.”

The authors also looked at how race interacted with these demographic trends. When analyzing White and Black Americans separately, they found that the widening fertility gap between the left and the right was primarily driven by White Americans. Among White participants, the reproductive advantage for conservatives grew substantially in the more recent birth cohorts.

For Black Americans, the researchers did not find a similar political divergence. While overall fertility among Black Americans also declined over time, birth rates remained relatively similar across the political spectrum. The authors suggest this might happen because ideological labels like liberal and conservative are often understood differently across various racial and cultural groups.

Beyond political views, the study found that other lifestyle factors strongly predicted family size. Education was consistently linked to lower fertility, meaning that individuals with more years of schooling tended to have fewer children. This negative association was particularly strong for women, a pattern that aligns with broader demographic research.

Religious attendance was positively associated with having more children. Interestingly, the data indicated that frequent religious attendance provided a stronger reproductive boost for men than it did for women. Even so, as the reproductive advantage of right-wing politics increased in recent generations, the independent effect of religious attendance on family size weakened slightly.

The authors suggest that differing views on family planning and lifestyle autonomy might play a role in these divergent birth rates. “The main takeaway is that political change is not driven only by persuasion, elections, or short-term social trends, but also by demographic processes,” Fieder said.

“Our findings suggest that differences in fertility rates across the political spectrum may gradually change the political composition of the US population over time,” Fieder said. “In particular, the decline in overall fertility appears to be driven largely by the sharp decrease in births among individuals left of the political center, possibly since the widespread use of modern contraceptives, although our data cannot directly prove this mechanism.”

This points to a significant limitation of the research. The study relies on observational data, meaning it cannot prove that holding a left-wing attitude directly causes a person to have fewer children. The survey data did not track individual fertility intentions or contraceptive use.

“We hypothesize that differences in the use of modern contraceptives contributed to the dramatic decline in fertility among individuals left of the political center,” Fieder said. “However, our data do not allow us to directly test or confirm this hypothesis.”

Additionally, measuring political orientation on a simple left-to-right scale oversimplifies the complex nature of human ideology. People often hold a mix of socially conservative and economically liberal views, which might influence their life choices in different ways.

Future research could explore how different types of conservatism, such as economic versus social conservatism, affect family planning. The researchers also plan to examine how these patterns operate outside of wealthy, post-industrial nations.

“We would also like to examine whether similar trends are beginning to emerge globally, particularly in developing countries, where our 2018 study suggested that fertility rates across the political spectrum were still relatively balanced,” Fieder said.

The study, “Falling Fertility on the Left as Key Driver of US Birth Decline,” was authored by Martin Fieder and Susanne Huber.