1 Department of Psychology, Concordia University, Montreal, Quebec, Canada.
2 Instituto de Estudios en Educacion, Universidad del Norte, Barranquilla, Atlantico, Colombia.

The transition to kindergarten can be stressful as children adjust to novel separations from their caregivers and become accustomed to their peer group. A 9-month study of 96 children (M_age = 5.37 years, SD = 0.42) from Barranquilla, Colombia, assessed socioeconomic differences in hypothalamic-pituitary-adrenal (HPA) axis functioning across the kindergarten year. Children were from four different classrooms in one school. Saliva samples were collected twice a day across 3 consecutive days at the beginning and end of the school year. We examined whether change in HPA axis activation across the year varied as a function of a child's socioeconomic status (SES) and experience in the peer group. We found that rejected children and lower SES children had lower cortisol levels early in the morning. Rejected children had a flatter morning cortisol slope. Lower SES children had higher cortisol than their higher SES peers at the end of the school year and a flatter morning cortisol slope. Taken together, these findings suggest that diurnal cortisol in children beginning kindergarten may be influenced by both peer rejection and SES.