To explore the relationship between the hypothalamic pituitary adrenal (HPA) axis and depression Hypotheses Abnormal HPA activity at 15 years is a risk factor for subsequent depression (at 18 years.) Exposure Abnormal HPA activity at 15 years (as approximated by cortisol awakening response and diurnal cortisol secretion.) Outcome Variable Depression at 18 years diagnosed by CIS-R. Confounding Variables Potential confounding factors are depression or anxiety at 15 years or previously, antenatal and postnatal depression, antenatal anxiety, lifestyle measures (sleep, fitness, and alcohol and illegal drug use,) body mass/ adiposity, gender, family history of depression, medications, smoking, stress (as approximated by socioeconomic status, significant life events and bullying), household composition, other psychiatric diagnosis or medical illness, and social support.