Study Finds Twins Catch Up to Singletons in Language by Age 7

A study analyzing data from 851 twin pairs and their younger singleton siblings in the United Kingdom found measurable differences in early childhood development. The research, published in the journal Child Development, reported that while twins initially scored lower in language, cognition, and social-emotional skills, they caught up to their singleton siblings in language by age 7.

The study's authors suggest the findings highlight factors unique to the twin experience.

Study Design and Methodology

The research was conducted by a team led by researchers from the University of York, with lead author Emily Wood, a developmental psychologist from King's College London. It utilized data from the Twins Early Development Study (TEDS), collected between 1996 and 2004.

• Participants: The study tracked 851 pairs of twins and their younger singleton siblings from the same families.
• Assessment Ages: Children's development was measured at ages 2, 3, 4, and 7.
• Assessment Methods: At ages 2, 3, and 4, data was collected via parent questionnaires. At age 7, assessments were conducted directly with the children via telephone.
• Developmental Domains: Progress was tracked across three areas: language skills, cognitive abilities, and social-emotional development.

The researchers noted this within-family comparison design is intended to account for shared confounding factors such as genetics, household environment, and socioeconomic variables. They stated that previous research on developmental delays in twins has produced mixed results.

Key Findings

The analysis revealed distinct patterns of development between the twin and singleton siblings across the measured domains.

• Cognitive Development: Singleton children scored higher than their twin siblings in cognition tests at all measured ages—2, 3, 4, and 7.
• Social-Emotional Development: Singleton children also exhibited higher scores in social-emotional development across all ages. They showed more prosocial behaviors and fewer reported conduct and emotional problems. The study reported that differences in specific areas, such as hyperactivity and peer problems, increased as children reached school age.
• Language Development: Twins scored lower than singletons in language skills during early childhood (ages 2-4). However, by age 7, twins had caught up to and then exceeded their singleton siblings in this domain.

The researchers described the observed disadvantages for twins in early childhood as "small to medium" in effect size but statistically significant.

Potential Explanatory Factors

The study's authors cited several factors that may influence the developmental patterns observed in twins.

• Shared Parental Resources: The researchers noted that twins must share parental attention, toys, and other resources from birth. Emily Wood stated, "When you have two children of the exact same age, they are in direct competition for everything – from toys and food to a parent's one-on-one attention."
• Parenting Stress: The study suggests parenting twins can involve greater emotional, physical, and financial demands compared to parenting singletons.
• Language Environment: The researchers cited previous evidence indicating that such strains can affect parent-child interactions, potentially resulting in parents directing shorter and less sophisticated speech toward twins.
• Social and Identity Factors: The study notes that twins share companionship from conception and are often identified socially as a pair rather than as individuals, which may affect social interactions with other children and each child's sense of individual identity.

Context and Implications

The researchers concluded that the findings suggest twins may benefit from additional support in their early years to mitigate developmental delays in skills that can influence life outcomes. They noted that failing to address early learning gaps could contribute to long-term challenges.

The study also contextualizes its relevance by noting that twin and multiple births are becoming more common in many countries, a trend attributed to factors like increasing maternal age and the use of assisted reproductive technologies such as IVF.