Temperament broadly refers to individual differences in behaviour that are typically measurable in infancy and early childhood. Broad dimensions include domains such as emotionality, negative affect, sociability and surgency. Measures typically capture a large number of individual subscales, as well as more general overall domains.

Fortunately, measurement of infant temperament has been developed over several decades, with considerable number of psychometric studies to support the measures and with an emphasis on capturing reliable individual differences. Commonly used scales include the infant temperament scales by Carey and the Infant–Toddler Social and Emotional Assessment by Carter and Briggs-Gowan.

Temperament and developmental milestones reflect early development of personality and behaviour. Infant temperament and milestones predict a variety of later outcomes in childhood.

Twin heritability for temperament domains has tended to be reported as between 30-40%. In general, this research field is characterised by smaller twin studies compared to studies of older ages. Some of the very large developmental twin cohorts of >5000 pairs (e.g. TEDS, CATSS) have either tended to begin main assessments after infancy or have included a small number of items in infancy.

Our study aims to explore the role of genetic variants on infant temperament and developmental milestones using a variety of state of the art statistical genetic methodology.