Figure 3.

Simulated data for a common genetic variant present in 30% of the population that depresses language z-score by 0.4. In addition, language scores are depressed by 0.4 z in those with bilateral speech. The genetic variant and bilateral speech have independent origins, but their effects are additive. The rate of bilateral speech is 9% in those with unimpaired language (z-score higher than −1.5) but 19% in those with language impairment (z-score less than or equal to −1.5). If the simulation is modified to give an interaction between genetic risk and bilateral speech, then the association between bilateral speech and language impairment becomes even stronger.