What is the difference between determinism and reductionism?

Reductionism is when complex behaviour is reduced to just one or two factors. For example, Behaviourism reduces complex learning down to just stimulus-response (e.g. classical conditioning), and the Biological approach reduces human behaviour down to the action of genes and neurons. The opposite of reductionism can be seen in the Humanistic appraoch - holism; whereby 'the whole' indivdiual is investigated to understand behaviour.

Determinism is when our behaviour is said to be determined by factors (i.e. not by our own free will). For example, the Psychodynamic approach states that human behaviour is driven by our unconscious minds (e.g. the id) - our conscious mind cannot control this, it can only rationalise it. Again, the opposite of determinism can be seen in Humanism - free will; whereby humans are active agents who determine their own development.