This is what I believe is referred to as "supression" in regression, where the correlation correlation between the independent and the dependent variable turns out to be of one sign whereas the regression coefficient turns out to be of the opposite sign.
Read here about supression: http://www.uvm.edu/~dhowell/gradstat/psych341/lectures/MultipleRegression/multreg3.html HTH -- View this message in context: http://r.789695.n4.nabble.com/Total-effect-of-X-on-Y-under-presence-of-interaction-effects-tp3514137p3516446.html Sent from the R help mailing list archive at Nabble.com. ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.