People with a severe traumatic brain injury (TBI) often experience problems understanding non-literal utterances such as sarcasm and lies in dyadic exchanges. This study aimed to investigate whether these problems extend to settings where speakers vary in their degree of sincerity and whether such problems are associated with deficits in social cognitive abilities (emotion perception, theory of mind, and self-reported empathy) or cognitive abilities (abstract reasoning, working memory, processing speed, attentional switching).Thirty-one adults with severe TBI (24 males) and 25 demographically matched controls (20 males) participated. They watched video vignettes depicting four actors volunteering for additional duties. Each speaker made a limited verbal response which literally suggested a willingness to be involved, but the sincerity with which the response was made was tempered by the actor's emotional demeanour. Participants rated each speaker in the vignettes for degree of sincerity (0-100%). Standardized measures of cognitive and social cognitive function were also taken.Control participants had excellent agreement (α = .90) in their rankings of actors according to sincerity. TBI participants were less consistent (α = .65). Overall, they were sensitive to decreasing sincerity but generally less accurate than control participants. They were poorer at differentiating between levels of sincerity and rated insincere expressions as more sincere, although they rated sincere expressions similarly. Poorer working memory and poorer social cognition were associated with poorer sincerity/sarcasm detection in the participants with TBI, but only social cognition was uniquely associated.Some adults with TBI have difficulty assessing the level of sincerity of speakers. Moreover, poorer social cognition abilities are associated with this difficulty.