Both international and national studies show an increasing discrepancy between legal reasoning in court practice and reasoning standards in legal theory. A comparative Belgian study establishes that while the reasoning structure in legal decision making according to legal theory ought to be unidirectional, from premises to conclusions, judges seem to first choose outcome of their decision, only after which they use various legal methods as technical tools to justify the already chosen decision. A Swedish study states that it is even usual that judges handle theoretically separate questions as if they were intertwined, in spite of the fact that separate reasoning is considered to be a necessary condition of the rationality of legal argumentation. The revealed tendency of increasing discrepancy between theory and practice is problematic not only from the point of view of legal science but even for parties in lawsuits, because the more the reasoning deviates from theoretical rationality the less foreseeable are the decisions. The purpose of the project is to identify factors that are causal to the deviating practices. According to pilot studies there is a connection between practical legal reasoning and cognitive psychological reasoning heuristics, which are known as sources of errors in reasoning. Legal reasoning in theoretically dubious cases will therefore be analyzed with the help of cognitive psychological theories of decision making heuristics.