ABSTRACT

One of the most important ways in which psychology differs from physics and the natural sciences in general arises from the existence of individual differences. Two litres of hydrogen that are treated identically respond ident­ ically, but any two human beings, even identical twins, are likely to respond quite differently to identical stimuli of many types. The reason for this is that people differ from one another, not only in appearance (that is, physically), but also in behaviour (that is, psychologically). Psychological differences between people have been investigated since ancient times (see chapter 1.1, Andrew M. Colman, and chapter 1.2, Raymond E. Fancher) and some progress has been made in understanding them.