Cognitive and verbal abilities of 24- to 36-month-old siblings of children with autism.

Researchers compared toddlers who have an older autistic brother or sister with same-age kids who do not. They looked at both thinking skills and language skills in children aged 24 to 36 months.

The team used standard tests to measure how well the children understood words and how well they spoke. They also gave basic cognitive tests to see if overall thinking ability differed between the groups.

The toddler siblings understood fewer words and spoke fewer words than the control group. Their scores for receptive and expressive language were lower.

Surprisingly, the two groups scored the same on non-verbal cognitive tests. A small slice of the sibling group had very low language scores, more than two standard deviations below the mean.

Garrido et al. (2017) pooled many studies and found the same medium-sized language gap from 12 to 36 months. Their meta-analysis includes the 2007 data, so the result is not a one-off.

Chuthapisith et al. (2007) seems to disagree. They tested slightly older preschool siblings and found no verbal IQ difference once the autistic child's own IQ was controlled. The clash disappears when you notice age: language lags show up first in toddlers, then may narrow by preschool.

Meier et al. (2012) followed similar children to age five. Standard language tests looked normal, but natural conversation samples still caught subtle social-language problems. Together, the papers trace a line: early toddler delay → later catch-up on tests → lingering real-life gaps.

Start screening language in baby siblings as soon as they reach their second birthday. Low scores at 24 months can flag the need for speech therapy before preschool. Even if scores later climb into the average range, keep checking real-life talking and listening during play. A quick language sample in your next assessment can catch the gaps that standardized tests miss.