Birth order effects on nonverbal IQ scores in autism multiplex families.
In families with several autistic children, the second-born sibling tends to score lower on nonverbal IQ—so factor birth order into both research matching and teaching plans.
01Research in Context
What this study did
Allen et al. (2001) looked at IQ scores in families with more than one child with autism. They asked whether birth order changes nonverbal IQ. They used standard IQ tests and compared first- and second-born siblings who both had autism.
What they found
Second-born siblings scored lower on nonverbal IQ than their first-born brothers or sisters. Social and language scores stayed the same no matter the birth order. The drop only showed up on the nonverbal part of the test.
How this fits with other research
Banach et al. (2009) studied the same families and added gender. They found no extra IQ split between boys and girls once birth order was counted. This keeps the spotlight on birth order, not sex.
Guisso et al. (2018) looked at a different group—Lebanese kids in a case-control study. They saw that first or second birth order lowered the odds of getting an autism diagnosis. That sounds opposite, but they studied risk of diagnosis, not IQ after diagnosis. Different question, different answer.
Billings et al. (1985) first showed that autistic girls cluster at the lowest IQ levels. Allen et al. (2001) shift the lens from sex to birth order, showing family place also shapes ability scores.
Why it matters
When you pick kids for genetic studies or plan skill-based programs, note if they are first- or second-born. A lower nonverbal IQ in the younger affected sibling could mean they need more visual or hands-on teaching. Check birth order before you set targets or match pairs in research.
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02At a glance
03Original abstract
Lord (1992) published a brief report showing a trend for decreasing nonverbal IQ scores with increasing birth order in a sample of 16 autism multiplex families, and urged replication in a larger sample. In this report, analyses of nonverbal IQ scores for a sample of 144 autism multiplex families indicated that nonverbal IQ scores were significantly lower in secondborn compared with firstborn siblings with autism. This birth order effect was independent of gender as well as the age differences within sib pairs. No such birth order effects were found for social or communicative deficits as measured by the Autism Diagnostic Interview-Revised (ADI-R), but there was a modest tendency for increased scores for ritualistic behaviors for the firstborn sibs. Further, there were no gender differences on nonverbal IQ scores in this sample. Results are discussed in terms of implications for genetic studies of autism.
Journal of autism and developmental disorders, 2001 · doi:10.1023/a:1012217807469