Early social and emotional communication in the infant siblings of children with autism spectrum disorders: an examination of the broad phenotype.

The team watched 6- to 18-month-old baby brothers and sisters of kids already diagnosed with autism. They compared these "ASD-sibs" with babies who had no family history of autism.

Researchers filmed each baby during play with a parent. They counted smiles, joint attention looks, and the way babies asked for toys.

ASD-sibs smiled less, started fewer shared looks, and responded to fewer pointing games. They also made fewer high-level requests, like showing or giving objects to share interest.

The gaps were small but showed up as early as six months.

Garrido et al. (2017) pooled many studies and found the same language lags continue into the second and third year. Their meta-analysis includes the 2007 data you just read.

Nevin et al. (2005) saw similar social-communication delays using the same play test one year earlier. The 2007 paper adds emotional smiles and more age points.

Chuthapisith et al. (2007) seems to disagree: they found no verbal IQ gap in preschool siblings. The difference is age. Tiny babies show small social-emotional lags, but overall language IQ can still catch up by preschool unless the autistic brother or sister also has intellectual disability.

You can spot risk before the first birthday. During early-intervention intake, ask parents about autism in older siblings. Then watch for fewer shared smiles, late pointing, and weak back-and-forth looks. These micro-signs do not diagnose autism, but they flag a baby who needs close tracking and may benefit from extra social games and parent coaching right away.