Introduction

Genetic influences on psychological functioning are well established (McGuffin et al., 2004). A multifactorial interactional model between polygenic inheritance and environmental experience is most commonly proposed. However, many single gene anomalies have characteristic psychological profiles.

The term ‘behavioural phenotype’ (O'Brien, 2002) describes aspects of psychological functioning attributable to a discrete underlying genetic anomaly, even when other contributors such as age, gender and social background are accounted for. The first published use of the term was by Nyhan in his proposal of an association between the inborn error of metabolism and severe self-mutilation in Lesch–Nyhan syndrome (Nyhan, 1972). Nyhan also reported behaviours characteristic of Cornelia de Lange syndrome including self-injury, hyperactivity and autistic features (Berney et al., 1999).

Somewhat earlier, Langdon Down described characteristic personality traits in individuals with Down syndrome including strong powers of imitation, a lively sense of humour, obstinacy and amiability. Research confirms greater similarity of personality and temperament between people with Down syndrome than expected by chance (Nygaard et al., 2002). It also confirms increased risks of Alzheimer's dementia (Holland et al., 2000) and depression (Collacott et al., 1998).

Clinical considerations

Information on psychiatric, psychological and behavioural functioning can be categorized into:

• intellectual functioning

• speech and language

• attentional deficits and impulse control

• social functioning and understanding

• other behavioural disturbances

Intellectual functioning

Intellectual abilities in those with a particular condition can vary widely. People with Down syndrome usually have moderate-to-severe intellectual disability, while those with fragile X syndrome usually have mild-to-moderate intellectual disability (Cornish et al., 2004b).