Background Family socioeconomic status (SES) is an important source of child health disparities in the USA. Chronic stress is one way SES may impact children's physiology with implications for later health inequalities. These processes may work differently across childhood due to differences in exposure and susceptibility to stressors at different ages. We assess associations between family SES and one biomarker of chronic stress exposure—low-grade inflammation detected by elevated C reactive protein (CRP)—and evaluate differences in the associations by child age.

Methods We used nationally representative data from the National Health and Nutrition Examination Survey and Tobit regression models to estimate SES associations with CRP and the moderating effects of age for children age 2–18 years. Our sample was limited to CRP ≤10 mg/l to focus on low-grade inflammation (N=13 165).

Results Children whose parent had less than a high school degree had 35% higher CRP than those with a college graduate parent; and, poor children had 24% higher CRP than those with high family income, net of controls. When children's body mass index was accounted for, low education and poverty associations were reduced to 19% and 15%, respectively. Child age interactions were negative and significant for both parental education and family income.

Conclusions This study provides new evidence that SES is associated with low-grade inflammation in children, and that these associations may be particularly strong during early and mid-childhood. Future research should further our understanding of stressors related to low family SES that may lead to immune system dysregulation during childhood.