We investigate two aspects of the autism triad, communication and social difficulties, in relatives of specific language impairment (SLI) probands (with and without additional autistic symptomatology) as compared to relatives of autism spectrum disorder (ASD) and Down's syndrome (DS) probands. Findings involving 726 first degree relatives of 85 SLI, 99 ASD and 36 DS probands revealed a higher rate of communication difficulties in relatives of both subgroups of SLI probands compared to ASD and DS relatives. Similar levels of social deficits were found in relatives of SLI + ASD and ASD probands. There was a higher than would be expected rate (4.3%) of ASD, particularly in siblings of SLI + ASD probands. Communication and social deficits appear to breed true in SLI and ASD.