Two approaches to development
milestones - what makes people develop in similar ways
individual differences - what makes people develop individual differences

Type VS Trait
Type categorises someone into one of a limited set of character types
Trait views someone as varying on two dimensions

Temperament - consists of biologically rooted individual differences in behaviour tendencies that are present in early life and are relatively stable across various situations and over time(Bates 1989a)

The study of child temperament has focused on defining a limited number of traits and gathering data from large numbers to examine variation among them, but also attempts to incorporate types.

Dimensions of temperament
Although there is disagreement about the exact number of traits that define the diffences between children, Bates (1898) describes 3 broad categories of dimension...

Stability - the degree to which a characteristic is present in comparison to other people over a period of time
Continuity - the similarity in the structure of behavioural differences across age.

The emergence of temperament
There is developmental progression in the components of temperament that becomes apparent as the child gets older.

Abstract tendencies vs visible behaviours
temperament refers to a general tendency to behave in particular ways over a wide range of situations. It is not a visible feature of everyday behaviour, rather these behaviours area reflection of underlying temperamental differences.

Biological basis
Hinde (1989) noted that Buss and Plomin (1984) used a criterion of 'genetically based' to differentiate temperament form other individual differences. Hinde argues all behaviour has genetic basis.

Buss and Plomin suggest that high heritability might distinguish temperament from other individual differences which show lower heritabilities.

Buss and Plomin (1984) summarized heritability data from 4 twin studies and found that Emotionality, Activity and Sociability may have a significant genetic component, as correlations between identical twins were high, but insignificant for DZ twins

According to Hinde, twin studies can only indicate the extent to which variation between children in an aspect of temperament is attributable to to variation in genetic make-up or the variety in experiences in children's lives.