Not just mom and dad – older siblings may also enrich the youngest child's vocabulary, a new study from the Department of Psychology at the University of Oslo reveals.

— We found that children with older siblings had smaller vocabularies than their peers, but only up to a certain point. For children with three or more siblings, the vocabulary began to increase again.

That's according to Audun Rosslund, lead author of the study recently published in the prestigious journal Child Development, in which researchers examined the relationship between language development and family composition in more than 6,000 Norwegian children aged 1–3.

Rosslund points out that previous research has shown that children with older siblings often have smaller vocabularies than firstborns. A typical explanation has been that having more siblings results in fewer resources and less attention from parents for each child.

— Our study shows that the picture is more nuanced. It's not just about birth order, he says.

To explain how older siblings may transition from competitors for resources to becoming language resources themselves, the researchers examined the ratio of the number of children and “caregivers” in the household, regardless of whether these were parents or older siblings.

— We then simulated models to find the threshold that could best explain the variation in vocabulary. That is, the age threshold at which older siblings should be considered caregivers and resources providers, rather than competitors to their little sister or brother, Rosslund explains.

Girls are faster

This turned out to be in early adolescence, and interestingly, the study revealed that older sisters reached the threshold one to three years earlier than older brothers.

According to Rosslund, girls typically have somewhat faster language development than boys, and potentially earlier social and cognitive maturation.

— This, along with a potentially greater interest in assuming the caregiver role, may explain our findings.

The relationship between the number of children and caregivers in the family had a more significant impact on the variation in young children's vocabulary than birth order alone.

— The key takeaway here is that household dynamics are crucial, not birth order. What's important is that young children in need of linguistic support have an abundance of caregivers around them, whether these are parents or siblings, Rosslund says.

He suggests that parallels can be drawn from these findings to the number of caregivers children have in nurseries and kindergartens, referencing to staffing standards.

— Language development starts already before birth, but nurseries can play a significant role, and it's tempting to draw connections from our findings to a nursery context – suggesting that more caregivers per child are positively associated with vocabulary, he says, adding

— Many children spend most of their waking hours in nurseries and kindergartens, some already from 10-12 months of age, so it's obvious that these can be influential for anguage development, especially in situations where the home environment is less stimulating for various reasons.

In April this year, the Norwegian Directorate of Health issued "New Recommendation for Assessing Young Children's Language Development," advocating closer collaboration between health centres and nurseries to ensure children facing language difficulties receive early assistance.

Another recommendation is to provide parents with information and guidance on how they can create a rich linguistic environment at home, stimulate the child's language development by actively engaging with them, while also limiting screen time.

— This is perhaps not about screen use in itself, Rosslund comments.

— But rather that excessive screen time reduces opportunities for other, more interactive activities that can be more influential in fostering language development.

He and colleagues from the University of Oslo previously published a study showing a negative relationship between daily screen time and vocabulary in Norwegian two-year-olds, whereas there was a positive relationship between the frequency of shared reading and vocabulary in both one- and two-year-olds.

— The recurring theme in these studies is the importance of time, attention, and linguistic stimulation, something both parents and older siblings may contribute to, concludes Rosslund.