Is decision making subject to irrationality? Mathematics and economics indicate that irrationality exists. It is described as part of the Real, but, it is still perceived as a 'margin of error'. In order to show that the decision depends on irrationality whose impact should not simply be randomized, this article, in a first step, precisely describes the concept of irrational decision. The second part talks about contagion, which is intrinsically linked to irrational decision. It details its different measurement, while the last part describes an experimental study on the subject