The associations of only-child status and household pet ownership with the risk of attention-deficit/hyperactivity disorder (ADHD) are inconclusive, and the joint effects of only-child status and household pet ownership on ADHD have not been thoroughly investigated. A population-based study was conducted in 2021 involving preschool children aged 3–6 years attending kindergartens in Longhua District, Shenzhen, China. Parents were invited to complete questionnaires providing information on sociodemographic and family-environmental factors. ADHD symptoms were assessed using the 26-item Swanson, Nolan, and Pelham Rating Scale as reported by parents.

Findings: This study included 63,282 children, representing 72.6% of all preschool children in this district in 2021. Among them, 34.4% were only-child and 9.6% were identified as having ADHD. Only-child status was associated with an increased risk of ADHD. Compared to children without a pet (cats or dogs) at ages 0–3 years, pet ownership at ages only 0–1 year, only 1–3 years, and both ages were associated with increased odds of ADHD: 1.59, 1.58, and 1.66, respectively, after controlling for potential confounders. A significant interaction between pet ownership at only 1–3 years and only-child status was observed. Similar findings were observed when the analyses were performed separately for boys and girls.

Authors conclude both only-child status and household pet ownership are associated with an increased risk of ADHD; however, the detrimental effect of pet ownership appears to be mitigated among only children when pet exposure occurs at ages 1–3 years, providing new insight into reducing family-related risk factors of ADHD.