Developmental psychology: the study of human physical, cognitive, social, and behavioural characteristics across the lifespan. Cross-sectional design: used to measure and compare samples of ppl at diff ages at given pt. in time e. g. groups from 1-, 5-, 10-, and 20 year olds. Quick (don"t have to wait for the participants to age) Problem cohort effects: differences between people that result from being born in different periods. Longitudinal design: follows the development of the same set of individuals through time e. g. select a sample of infants and study them periodically over 20 years. Attrition; when participants randomly drop out a study. Change is fast during sensitive periods; when the child is most ready to learn. Sensitive periods: window of time during which exposure to specific environmental stimulation is needed for normal development of certain abilities e. g. need to be exposed to language in infancy. Zygotes to infants: from one cell to billions.