Autism spectrum disorder (ASD) is characterized by social communication deficits, impaired social interaction, and restricted and stereotyped behaviours and interests. The typical onset is during early childhood. Behaviourally, people with ASD have difficulties in tolerating proximity. Insecure attachment is frequently observed. At the cognitive level, people with ASD have selective difficulties in mentalizing or ‘theory of mind’, possibly related to a dysfunctional mirror neuron system and alterations of the oxytocin system. Conversely, many with ASD (particularly those formerly diagnosed with Asperger’s syndrome) have superior technical skills. It has been hypothesized that the brains of individuals with ASD are skewed to maleness, possibly linked to genomic imprinting of paternal genes. The preservation of genes that predispose to ASD may have undergone sexual selection fostering ‘slow’ life-history strategies. None of the evolutionary hypotheses on ASD is conclusive so far, but open to empirical testing.