&gt; For example when you study people they behave differently and furthermore as soon as they think that you think that they will behave in a certain way they may well behave differently<p>That&#x27;s not an inherent limitation for applying science. Signal theory has models that represent circuits with feedback loops, where the output ; similar models could be developed to study people who know are being studied.<p>Also, if you use data collections that were compiled for other purposes, you can study the behavior of people who don&#x27;t know are participating un such study. Big data is essentially that.<p>&gt; As you say this all derives from the biggest problem with all the social sciences people are not rational actors and any mathematical model has to encapsulate real human behaviour.<p>There are theories that don&#x27;t assume perfect rational agents, and can still predict behavior by studying the common human bias. See the work of behavioral economist Dan Ariely for an academic approach.