A hidden link between impulsivity and obesity may not be fixed in human biology but shaped by the cities we live in.
Using a novel engineering-based approach, researchers from NYU Tandon School of Engineering and Italy's Istituto Superiore di Sanità found that attention-deficit/hyperactivity disorder (ADHD) contributes to obesity not only directly through known biological pathways but also indirectly, by reducing physical activity. The findings are published in PLOS Complex Systems .
Obesity prevalence is also influenced by other city-level variables, such as access to mental health services and food insecurity, thereby opening the door to potential mitigation strategies.
To uncover the nexus between ADHD and obesity, the research team applied urban scaling laws — a mathematical framework from complexity science — to public health data from 915 U.S. cities. Urban scaling describes how features of cities change with population size, similar to how biological traits scale with body size.
They found that both ADHD and obesity decrease sublinearly with population: as cities grow, per-capita prevalence declines. Meanwhile, access to mental health providers and college education rises superlinearly, increasing faster than city size. Larger cities, it seems, offer not just more services, but disproportionately more support for conditions linked to impulsivity.
But size alone doesn't tell the full story. To reveal where cities over- or underperform relative to expectations, the researchers used Scale-Adjusted Metropolitan Indicators (SAMIs). SAMIs measure how much a city deviates from what urban scaling would predict — highlighting, for example, when a small city has unusually low obesity rates or when a large one falls short on mental health access. These deviations became the foundation for a causal analysis.
"Urban scaling and causal discovery methods allow us to see relationships that traditional health research might miss," explains Maurizio Porfiri , senior author on the PLOS paper. Porfiri is an NYU Tandon Institute Professor with appointments in the Departments of Mechanical and Aerospace Engineering, Biomedical Engineering, Civil and Urban Engineering, and Technology Management and Innovation. He also serves as Director of the NYU Center for Urban Science + Progress (CUSP).
"Without accounting for how city size naturally affects health metrics, we'd misattribute success or failure to the wrong factors. By filtering out these population effects first, we can identify the true causal pathways linking ADHD to obesity — and more importantly, how urban environments modify these relationships," adds Tian Gan
