The most ‘important and greatest puzzle’ we face as humans is ourselves (Boring, 1950, p. 56). Humans are a puzzle – one that is complex, subtle and multi-layered, and it gets even more complicated as we evolve over time and change in different contexts. When answering the question ‘What makes us who we are?’, psychologists put forward a range of explanations about why people feel, think and behave the way they do. Just when psychologists seem to understand one bit of ‘who we are’, up pops some new evidence to show a different side! It is not easy to pin down all the many influences. This study unit is just one of many that can be found on LearningSpace, part of OpenLearn, a collection of open educational resources from The Open University. Published in ePub 2.0.1 format, some feature such as audio, video and linked PDF are not supported by all ePub readers.