BACKGROUND: There is a pressing need for research on neurodevelopmental conditions to focus on predictors of resilient or positive outcomes, rather than core symptoms and impairment. One promising avenue is to consider whether child-parent similarity contributes to a protective family environment. For instance, investigations of the similarity-fit hypothesis have shown that parent-child attention-deficit/hyperactivity disorder (ADHD) trait similarity is associated with more favourable parent or child ratings of parenting and parent-child interaction. However, very little similarity-fit research has focused on autism, and none to date has investigated whether parent-child trait similarity is more broadly predictive of children's outcomes beyond parent-child interaction. We assessed whether parent-child autism and ADHD trait similarity predicted children's social functioning and psychological well-being in early childhood in a family history cohort. METHODS: Our analytic sample comprised 222 children (45.5% female) and their parents from a longitudinal family history (autism and/or ADHD) cohort. A novel parent-child trait similarity measure was computed for autism and ADHD traits in each parent-child pair, and robust hierarchical regression was used to assess whether mother-child and father-child autism and ADHD similarity predicted children's social functioning and psychological well-being at age 3 years, after accounting for the main effects of parent and child traits. RESULTS: Mother-child autism trait similarity positively predicted both social functioning and psychological well-being in children, while mother-child ADHD trait similarity positively predicted children's social functioning (but not well-being). Furthermore, father-child autism trait similarity positively predicted children's social functioning, though it fell just short of statistical significance in outlier-robust regression. CONCLUSIONS: Our findings suggest that parent-child neurodevelopmental trait similarity may act as a protective or promotive factor for children's early social functioning and psychological well-being. Further work is warranted to determine whether there are similar effects in later childhood and to investigate the potential mechanisms underlying similarity-fit effects on children's outcomes.