Changes in Family Composition and Marital Status in Families with a Young Child with Cognitive Delay Chris Hatton*, Eric Emerson* ,  , Hilary Graham à , Jan Blacher § and Gwynnyth Llewellyn   *Lancaster University, Lancaster, UK;  University of Sydney, Sydney, NSW, Australia; àUniversity of York, York, UK; § University of California at Riverside, CA, USA Accepted for publication 26 September 2009 Background Few studies have investigated parental separation, partnering and re-partnering among popula- tion-based cohorts of children at risk of intellectual or developmental disabilities. Methods Secondary analysis of data extracted from Waves 1–3 of the UK’s Millennium Cohort Study. Infor- mation on the residence status of parents at Waves 1, 2 and 3 was available for 10 848 families. Results Children with early cognitive delay (ECD) were significantly less likely than other children to be living in households with both biological parents or in households where the mother was married at all time points, including at the time of the child’s birth and when children were aged 9 months, 3 years and 5 years. Although families with a child with ECD were more likely than families with a typically developing child to experience changes in both family composition and marital status over the first 5 years of the child’s life, the net effect of these changes in family composition and marital status from the time of the child’s birth to the time the child was 5 years old was similar for both groups of families. The increased levels of family change amongst families with a child with ECD were wholly accounted for by differences in family socio-economic circumstances. Conclusions More attention needs to be paid to describ- ing and explaining the dynamics of family composition in the early years of children with intellectual and developmental disabilities. ‘Stress reaction’ models attempting to account for rates of parental separation need to incorporate socio-economic factors. Keywords: children, divorce, families, separation Introduction The nature and composition of families has changed markedly in most of the world’s high-income countries over the past 50 years. These changes have been char- acterized by increasing rates of cohabitation, decreasing rates and delay of marriage, increasing rates of divorce and family dissolution and increasing rates of children living in single parent households (Cherlin 2005; Office for National Statistics 2008). In the USA for example, the percentage of families with children headed by lone parents rose from just over 10% in 1970 to just under 30% in 2003 (Cherlin 2005), with current esti- mates suggesting that over 50% of US children are likely to live apart from one of their parents before they reach the age of 18 years (Bumpass & Lu 2000). In the UK, the percentage of children living in single parent households rose from 7% in 1972 to 23% in 2007, with all of this rise being due to increases in lone mother headed households (Office for National Statis- tics. 2008). In the USA, nearly 50% of marriages are likely to end in divorce within 20 years (Cherlin 2005). In the UK, the current population-based divorce rate has risen to 50% of the marriage rate (Office for National Statistics. 2008). There is a now a substantial body of research exam- ining the impact of parental separation and ⁄ or divorce on developmental outcomes for children (Rodgers & Pryor 1998; Amato 2000, 2001, 2005; Barber & Demo 2006; Conger & Donnellan 2007). Overall, this research suggests that the negative impacts of parental divorce and dissolution on children may be limited and fairly Journal of Applied Research in Intellectual Disabilities 2010, 23, 14–26 Ó 2009 The Authors. Journal Compilation Ó 2009 Blackwell Publishing Ltd 10.1111/j.1468-3148.2009.00543.x Published for the British Institute of Learning Disabilities