Why was this study done? This study examines residential surrounding green space in association with intelligence and behavior in a study area that includes a spectrum of urban to rural environments. Previous studies mainly focused on urban areas, whereas only a few studies have explored differences in the effect of green space on cognition in nonurban settings. Understanding the health disparities that exist between urban and rural environments is essential for maintaining and improving human well-being in a rapidly urbanizing world. What did the researchers do and find? This longitudinal birth cohort study of twins assessed intelligence in 620 children between 7 and 15 years old. Our results indicate that residential green space is especially beneficial for intellectual and behavioral development of children living in an urban environment. What do these findings mean? We show that low residential green space in urban children is associated with a "shift" towards a higher incidence of low IQ demonstrating the public health impact of our findings. Background Exposure to green space has beneficial effects on several cognitive and behavioral aspects. However, to our knowledge, no study addressed intelligence as outcome. We investigated whether the level of urbanicity can modify the association of residential green space with intelligence and behavior in children. Methods and findings This study includes 620 children and is part of the East Flanders Prospective Twin Survey (EFPTS), a registry of multiple births in the province of East Flanders, Belgium. Intelligence was assessed with the Wechsler Intelligence Scale for Children-Revised (WISC-R) in 620 children (310 twin pairs) between 7 and 15 years old. From a subset of 442 children, behavior was determined based on the Achenbach Child Behavior Checklist (CBCL). Prenatal and childhood residential addresses were geocoded and used to assign green space indicators. Mixed modeling was performed to investigate green space in association with intelligence and behavior ...