Double trouble: Study reveals early developmental gaps in twins compared to siblings

Posted on Wednesday 25 March 2026

The research, by the University of York, highlights a disadvantage for twins in early development, but finds they leapfrog their singleton siblings in language skills by age seven.

The researchers emphasise that these differences are a result of the unique environment for twins - such as sharing parental attention and resources - rather than a reflection of parenting quality. 

Twin experience

The study, funded by the Nuffield Foundation and published in the journal Child Development, analysed data from 851 families in the Twins Early Development Study (TEDS).The study is the first to compare twins to their younger, single-born siblings within the same household. This approach allowed researchers to isolate the twin experience from other factors like income and education that often skew developmental research.

The researchers found that twins scored lower than their singleton siblings in language, cognition, and social-emotional development at ages two, three, and four. While many of these gaps persisted until age seven, a dramatic reversal occurred in verbal ability, with the findings suggesting that twins significantly outperform their singleton siblings in language tests after that age. 

Despite the language catch-up, twins continued to show lower scores in cognitive and social-emotional areas, with peer-related difficulties appearing to widen as they reached school age.

Often overlooked

Lead author of the study, Emily Wood, a postgraduate researcher from the Department of Education at the University of York, said: “The twin experience creates a specific set of challenges that are often overlooked. 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. This is a challenge inherent to being a twin, and it's something parents of single children don't have to navigate in the same way.

“Our research suggests that because the rearing environment is so demanding for twin families, these children may benefit from targeted support in their early years to help bridge the gap before they start school.”

Surprising

Senior author of the study, Professor Sophie von Stumm from the Department of Education at the University of York, added: “Our results regarding social-emotional development are particularly surprising. There is a common assumption that twins might have an advantage here because they grow up with a built-in companion, which could lead to better sharing skills or higher empathy. However, we found the opposite: twins actually reported more peer problems than singletons as they got older. It suggests that growing up as a pair doesn't necessarily translate to easier socialisation with others.”

The findings come at a time when twin births are on the rise in Western populations. Researchers point to the fact that women are, on average, older when they have children, alongside an increase in the use of in-vitro fertilisation (IVF), both of which are linked to higher rates of multiple births. More sophisticated medical care also now means that twin pregnancies – which are often higher risk – are monitored more closely than ever, leading to much higher rates of survival into infancy.

The study analysed longitudinal data from children who were observed over a five-year period. While the variability within the groups was comparable – meaning twins are just as able as singletons to reach high levels of achievement – the mean or average scores showed a clear distinction: In the early years, twins showed a lower performance across domains. 

Extra support

The researchers hope these findings will encourage a shift in how healthcare and education providers view twin development. Currently, support is heavily front-loaded toward pregnancy and birth.

“We want to raise awareness that being a twin comes with additional challenges that don't just vanish after birth,” adds Professor von Stumm. “To address these challenges, twins could be offered the extra support they might need in cognition and social play to ensure they aren't starting school at a disadvantage.”