Study of change & stability of human physical, cognitive, social, & behavioural characteristics across the life span. Approach to developmental psych; follows the development of same set of individuals through time. E. g identify set of 50 infants (cid:0) 20 yrs. cognition. Generation ; consequences of being born in a particular year or a narrow range of yrs. 1st phase of prenatal development; from conception to 2 wks. After implantation, blastocyst develops into an (1) embryo + (2) placenta. @ diff. ages at a given point in time. E. g. comparing ppl from diff. age groups born prematurely. May occur w/ longitudinal design returning mail/calls, become ineligible, Window of time during which exposure stimulation is needed for normal development. Estimated time since fertilization; can be measured has developed. A cell formed by the fusion of a sperm & an ovum (egg cell); marks the beginning of germinal stage. This will move along fallopian tube developing into embryo & placenta.