A common definition of "binge drinking" is consuming 5 or more drinks at one setting for men, and 4 or more for women. An observational study finds that students who binge drink have lower GPAs, on average, than those who don't. However the possibility of confounding means that we can't conclude that increased alcohol consumption causes lower GPA. Select any possible lurking variables that may be confounding the relationship between binge drinking and GPA: