“The most salient finding was that children severely deprived up to 6 months and non-deprived UK adoptees had similarly low levels of problems, whereas severe deprivation lasting 6 months or more had persistent detrimental effects on individuals’ behavioural and social development in terms of symptoms of autism spectrum disorder, disinhibited social engagement, and inattention and overactivity through to young adulthood (pooled p<0·0001 for all), despite the fact that they were raised in supportive and caring adoptive families. Compared with the UK controls, the high deprivation group also had a higher proportion of people with low educational achievement (p=0·0195), unemployment (p=0·0124), and mental health service use (<11 years p=0·0120, 11–14 years p=0·0032, and 15–23 years p=0·0003). The longitudinal comparison across the four assessment waves revealed two interesting findings that would have stayed unnoticed if only young adult data had been used. The higher rates of cognitive impairment in the high deprivation group at ages 6 years (p=0·0001) and 11 years (p=0·0016) compared with the UK controls remitted to normal rates at young adulthood (p=0·76). By contrast, emotional problems in the high deprivation group (depression and anxiety) showed a late onset pattern with a significant increase in levels from ages 11 and 15 years to young adulthood (p=0·0005).