Why are we making purchases we do not need? How is it that in some situations, we made decisions inconsistent with our preferences displayed? It's (really?) I decide is a book written by an expert in behavioral economics, recent discipline that borrows from psychology and economics. The author starts from the premise that we tend to overestimate the conscious part of our choices. Indeed, classical economics theory emphasizes the rationality of the individual: we are all capable of making decisions that benefit us through our reasoning. On a daily basis, so we compare the choices available to us before choosing the most beneficial. But behavioral economics shows that this is far from true. We regularly take imperfect decisions or irrational when compared to our tastes and objectives displayed. Must we conclude that man is incomprehensible? In fact, Dan Ariely shows in his book that this irrationality is neither senseless nor random. It is recurrent and repetitive otherwise. It lies in the functioning of our brain and a number of stable bias. Throughout the chapters, he gives us the result of much research and experience that will prove very useful to the marketer. First, we discover that the brain does nothing in absolute values, so that we can significantly influence the value placed on an offer by playing on comparables that we provide to the customer. The attraction exerted almost irresistible free, and beyond any objective justification, is also instructive. Finally, the influence of our expectations of how we live our consumer experience, if it contains the well-known theories of the placebo effect, provides many avenues for better use of their offer.