As with the NFP programme, the stimulation programme improved internalising behaviour (Walker et al., The stimulation intervention also improved earnings by 33% at age 22 (Gertler et al., The Perry Preschool programme enriched the lives of 3- and 4-year-old low-income black children with initial IQs below 85 at age three.11 They attended two-and-a-half-hours of centre-based preschool five days a week for two years. In addition, home visits promoted parent-child relationships. The programme ended after two years of enrolment, and both treatment and control groups entered the same school.