Introduction

In writing about behavioural phenotypes, we can see just how far the field has come in only a few short years. In the first edition of this volume, published in 1999, we began our chapter with the paradox that geneticists had discovered over 750 different genetic disorders associated with intellectual disabilities (ID), but that behavioural work lagged far behind. We lamented the so-called ‘two cultures’ of behavioural work in ID (Hodapp & Dykens, 1994), noting how researchers who were more biomedically oriented versus more behaviourally oriented rarely intersected one with another.

But much has changed over the past five to ten years. Consider the sheer number of research articles on behaviours of the most prominent genetic disorders of ID. Using computer searches comparing the 1980s to the 1990s, one sees remarkable, almost exponential, increases. From the 1980s to the 1990s, the numbers of behavioural research articles on Williams syndrome increased from ten to 81; on Prader–Willi syndrome from 24 to 86; on fragile X syndrome from 60 to 149. Even in Down syndrome, the sole aetiology featuring a longstanding tradition of behavioural research, the amount of behavioural research almost doubled — from 607 to 1140 articles — over these two decades. The years since 2000 have shown even more pronounced increases, particularly with regards to Williams and Prader–Willi syndromes.

Granted, such progress is uneven, with many more studies performed on Down syndrome than on almost all other syndromes combined.