Build Baby Build is a provocative title for an economics paper. It also accurately describes the policy decision-makers should adopt to address the role housing affordability plays in falling fertility rates. The paper, subtitled How Housing Shapes Fertility, captures the research results of Benjamin K. Couillard, a PhD Candidate with the Department of Economics, who investigated the role of housing prices, location and family size using data from the U.S. Census and the American Community Survey. In his analysis of the data, Couillard uses a structural model to understand the relationship between housing costs and fertility. Building on existing quasi-experimental work, his approach directly accounts for the differing trade-offs that large and small families make when choosing where to live, a phenomenon economists call sorting.
“Housing and fertility are jointly determined because large and small families sort into different locations based on the number of children they have – or want to have – and housing costs,” he said.
Couillard’s research is a matter of immediate concern. According to the National Center for Health Statistics (NCHS), the fertility rate in the United States is 1.62 children per woman while the population replacement rate would be 2.1.
“If rents had stayed flat since 1990, there would have been 11% more children born in the U.S. into the 2010s,” Couillard said. “I also found that the total fertility rate in the 2010s would have been 77% closer to the replacement rate if rents hadn’t increased. That’s a surprising result. It suggests that housing costs are a major driver of fertility decline. Housing abundance is not just about affordability; it’s about long-term demographic sustainability.”
Couillard, whose research interests include urban economics and industrial organization (IO) combined aspects of each subdiscipline into his methodologies to build a model that separates other factors driving housing choices from costs and fertility decisions.
“It’s actually more accurate to call it a living arrangement model, where fertility is a key component,” he explained. “Agents can choose to start a family or remain with their parents, and the model captures dynamic considerations like life-cycle timing and long-term housing needs.”
To date, most housing policy actions have focused on increasing the supply of smaller units like studios and one-bedroom condos to help new home buyers to get on the bottom rung of the property ladder rather than climb it. Since US-based parents and would-be parents prefer that each child have their own room, Couillard’s results suggest subsidies and other policy-based interventions should focus on larger units.
“Ben develops and estimates a dynamic discrete choice model of demand for location, housing unit type, household formation, and fertility,” said Professor Nathanial Baum-Snow, Couillard’s thesis supervisor. “Model counterfactuals reveal new insights about the role of housing costs in driving the marked decline in fertility over the past two decades. Current US fertility rates would rise markedly with lower cost 3+ bedroom housing units; while subsidizing the construction of additional one bedroom housing units would do little to address demographic sustainability.”
In other words, people will have more children if they could accommodate them comfortably.
“If we can get housing costs down, that’s good for affordability, but it also helps us avoid the demographic problems that come with aging populations and declining birth rates,” Couillard said. “A maximalist housing policy, one that aggressively expanded supply to prevent costs from rising, could have solved the majority of the fertility problem.”
On the job market in 2025-26, Couillard has been thinking about how his work reflects his views of the role of economics and scholarly research in society.
“I care about making my work meaningful and accessible, not just to specialists but to the broader public and university community,” he explained. “In times when there’s skepticism toward academia, showing that our work has real-world relevance helps build trust and engagement.”
That ethos is evident in Couillard’s work creating opportunities for his fellow PhD students. For five semesters, he was the primary organizer of the Student Structural Seminar Series (S^4) for five semesters and organized a related mini conference in August of 2025.
Accessible and meaningful material was also a priority in his work as a teaching assistant.
“I’ve had the pleasure of working with Ben as my TA for three courses, and I wish we had the chance to work together even more,” said Professor Yuanyuan Wan. “He has an incredible sense of responsibility for students and often goes out of his way to help less experienced TAs in parallel sessions. As for how good he is as an instructor. Whenever the department asks me to recommend PhD students who can teach an undergraduate applied econometrics course, his name is always the very first one I give.”
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