A 21-year-old who struggles with reading, anxiety, or finding work may be carrying genetic and environmental risks that have been building since infancy—and a new study will track 10,000 twins to pinpoint what predicts success or failure during this critical life stage. This matters because "emerging adulthood" is a vulnerable tipping point for lifelong mental health and cognitive problems, yet most research stops at adolescence. The gap is especially wide for people with low IQ (between 70 and 79), who are ten times more common than those with diagnosed intellectual disability but are rarely studied. Without knowing which factors—genetic, environmental, or both—drive poor outcomes, it is impossible to design effective early interventions. If the research succeeds, it could change how schools, health services, and social support systems identify at-risk young people before they struggle. For example, a child flagged for low literacy or anxiety at age 7 might receive targeted help that prevents a crisis at 21. The findings could also reshape how policymakers allocate resources for mental health and adult education, reducing long-term personal and societal costs. This is fundamentally a discovery science project. It will not produce a new therapy or app tomorrow. But by tracing the developmental roots of cognitive and mental health problems, it lays the groundwork for future prevention strategies—much as early twin studies of reading difficulties eventually led to classroom interventions now used worldwide.
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'Emerging adulthood', the phase of the life course between adolescence and full-fledged adulthood, is when young people transition to independence as they leave home and make their own way in the world; a vulnerable, pluripotent time that can be a 'tipping point' for life-long mental health problems. We will bring together genetic and genomic strategies in order to establish the genetic, environmental and developmental origins of cognitive and mental health problems in emerging adulthood. Our sample is unique, consisting of members of twin pairs who have been assessed longitudinally since infancy as part of the Twins Early Development Study (TEDS), which has been continuously funded by MRC programme grants (G0901245, G0500079, G9424799) since 1995. This is the largest ever in-depth longitudinal twin study. The twin design can disentangle genetic from purely environmental influences, and the infancy-through-adolescence data, including genome-wide genotypes, offer a developmental framework that can identify behavioural, genetic and environmental risk/protective factors that predict good vs poor outcomes in emerging adulthood. Our proposed research has two objectives: Objective 1. Identify behavioural, genetic and environmental risk/protective factors that predict good vs poor outcomes in emerging adulthood. We will assess a UK-representative sample of 10,000 TEDS twins (5,000 pairs) at age 21 on a broad battery of cognitive functioning (e.g., literacy and numeracy), mental health (e.g., anxiety and well-being) and environmental measures (e.g., emotional support and life events) comparable to the batteries used in our previous 20 years of assessment, as well as new measures specific to emerging adulthood. This will allow us to bring the longitudinal, genetically sensitive dataset of TEDS to bear on cognitive and mental health problems in emerging adulthood in the context of normal development. We will also conduct genomic analyses using genome-wide genotyping of millions of DNA markers for 5,000 unrelated participants (one member of a twin pair) and for 2,500 non-identical twins of these individuals; these resources will enable cutting-edge genomic analyses based on DNA alone. Objective 2. Identify predictors of good vs poor outcomes for individuals of low IQ in emerging adulthood. Diagnosed intellectual disability (<70 IQ) has received much attention in recent molecular genetics research, but little is known about low IQ (70-79 IQ), which is ten times more common and which also results in major personal and societal costs, especially during the tipping point of emerging adulthood in an increasingly technological world. For Objective 2, we will investigate the problems faced by young adults who are challenged cognitively as they make their own way in the world. Our work will yield the first genetically sensitive study of the effects of low IQ on adjustment in emerging adulthood, by assessing in person 600 TEDS individuals with 70-79 IQ scores at age 21 (members of 400 twin pairs with at least one low-IQ proband), co-twins from discordant pairs, and 200 controls. We will conduct intensive in-person interviews focused on the adaptive functioning and adjustment of these low-IQ individuals and their co-twins as they begin to make their way in a cognitively taxing adult world. Their outcomes will be compared to a representative control sample of 200 TEDS individuals assessed in the same way. In addition to describing outcomes for individuals with low IQ as they emerge into adulthood, we will use the twin design comparing identical and non-identical twins to investigate, for the first time, the genetic and environmental origins of these outcomes of low IQ. Identifying the genetic and environmental origins of cognitive and mental health problems in emerging adulthood and tracing their developmental roots is a fundamental step towards predicting and preventing these problems.
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