Generalized Additive (Mixed) Models (GAM(M)) - an overview
GAMs or GAMMs are used to fit and plot time varying functions to responses. They can include interaction terms to compare curves across different groups (e.g. males and females) with R^2 used as means of assessing the degree of fit (interpreted as in linear regression). GAMs and GAMMs can be fitted in R using the mgcv procedure. McKeown and Sneddon (2014) describe and illustrate GAMs and GAMMs with accompanying R code using the mgcv procedure.
Reference
McKeown GJ and Sneddon I (2014) Modeling Continuous Self-Report Measures of Perceived Emotion Using Generalized Additive Mixed Models. Psychological Methods 19(1), 155-174.