English Abstract: Behavioral Sciences are relevant to the Law because they have enhanced the comprehension and prediction of human reaction to norms and other interventions. However, they have not yet built a complete model of behavior apt to substitute the one assumed in mainstream economic research. Their contribution may be better seen, so far, as complementary.“Nudge”, as characterized by Thaler & Sunstein, works as a Weberian “ideal type”. Whether or not a real world intervention deserves to be named nudge is a gradual matter. Naming, nonetheless, is far from being an autonomous factor to justify any intervention. Real nudging, at least in a relevant sense, is never neutral or absolutely liberty preserving. Hence, justification of real policies and institutions, even properly qualifying as nudges, has to be found in accordance to external criteria. Efficiency goals are a criterion of that kind. In the field of Law, legal reasoning and argumentation are the standard ways to discuss those issues.In legal argumentation, Alexey's Weight Formula may give a formal framework to analyze the point. Within that scheme, “ideal” nudges will generally justify, because their relatively limited interference regarding their positive effect. More interestingly, the same framework allows systematically assessing those properties in real interventions. Additionally, behavioral knowledge shows decisive to give value to the variable that captures premises´ reliability in that formula.The role of behavioral findings is not limited to those segments of legal reasoning though. By means of a few examples, it is shown that they provide deeper (and crucial) insight into problems underlying various stages of legal reasoning. Hence, issues deserving behavioral consideration may arise in indeterminate matters, many of them included in the core of the legal profession. There is no pattern to predict their location within the process of legal reasoning. That indetermination makes it not possible for jurists to fully outsource the knowledge to solve those problems. Then, it makes some knowledge on the field necessary, even to discover the issues deserving deeper behavioral analysis. At least the knowledge to build the interface required to interact with experts in that field, then, should helpfully be included in the standard legal education
