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On Twitter, I was asked if there were serious research papers published on coffee consumption and labour productivity. There are some papers on coffee breaks and productivity, e.g. Productivity Through Coffee Breaks, but I could not find anything on coffee consumptions. Since I could not find any dataset with personal consumption (maybe I should start keeping tracks of my own consumption to run a study) I tried to find data for national consumption instead (even if we know that both are - clearly - not equivalent)last year, Sabine published on http://backreaction.blogspot.fr/ a dataset with consumption of coffee, per country (and per unhabitants),on http://en.wikipedia.org/ we can find a dataset with GDP per hour worked for some countries (which can be seen as a common  measure of the productivity of a country)If we merge those two datasets, we get&#62; base=read.table(+ "http://freakonometrics.free.fr/cafe.csv",+ header=TRUE,sep=";",dec=",")&#62; b=base[!is.na(base$GDP.PPP),]&#62; plot(b[,3],b[,4],xlab="Coffee Consumption",+ ylab="GDP per hour worked")&#62; text(b[,3],b[,4]+1.6,b[,1],cex=.6)&#62; library(splines)&#62; X=b[,3]&#62; Y=b[,4]&#62; B=data.frame(X,Y)&#62; reg=glm(Y~bs(X),data=B)&#62; y=predict(reg,newdata=data.frame(+ X=seq(0,10,by=.1)))&#62; lines(seq(0,10,by=.1),y,col="red")