Recent evidence suggests that psychopathy is a trait continuum. This has unappreciated implications for understanding the selective advantage of psychopathic traits. Although clinical psychopathy is typically construed as a strategy of unconditional defection, subclinical psychopathy may promote strategic conditional defection, broadening the adaptive niche of psychopathy within human societies. To test this, we focus on a ubiquitous real-life source of conditional behaviour: the expected relational value of social partners, both in terms of their quality and the likely quantity of future interactions with them. We allow for conversational interaction among participants prior to their playing an unannounced, one-shot prisoner's dilemma game, which fosters naturalistic interpersonal evaluation and conditional behaviour, while controlling punishment and reputation effects. Individuals scoring higher on factor 1 (callous affect, interpersonal manipulation) of the Levenson self-report psychopathy scale defected conditionally on two kinds of lowvalue partners: those who interrupted them more during the conversation, and those with whom they failed to discover cues to future interaction. Both interaction effects support the hypothesis that subclinical primary psychopathy potentiates defection on those with low expected relational value. These data clarify the function and form of psychopathic traits, while highlighting adaptive variation in human social strategies.

The work reported here was funded by a Wellcome Trust grant to Julie Dockrell and David Messer. We are grateful to Gillie Wilson for collecting part of the data presented here and all the children who participated. Two anonymous reviewers and the editor have provided detailed and constructive criticisms. 2 A proportion of the data on naming objects was presented at the Child Language seminar 1997 and published in the conference proceedings and a proportion of the comparative data between objects and actions was presented at the International Congress for the Study of Child Language 1999.

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There is increasing interest from therapists and researchers in children's word-finding difficulties (WFDs). Word finding difficulties are usually considered to be present when children are able to identify a referent from a set of exemplars, but have difficulty producing the target word when shown a picture or in conversation. Word finding difficulties are associated with a number of conversational forms such as delays in the production of a word, the use of long pauses within phrases, frequent use of place holders ('uh', 'um', etc.) and the use of circumlocutions. Although interest is being shown in WFDs, most of the data come from relatively small samples with the result that one knows little about the prevalence of the condition, what circumstances exacerbate the condition, the occurrence of associated difficulties and the types of therapy that are employed. To obtain this information a survey was carried out in the South-East of England of practitioners working with children who have language difficulties. Twenty-three per cent of children in language support services were identified as having WFDs. Most respondents used a mixture of formal and informal assessments. It was reported that WFDs were associated with difficulties in grammatical production, word meaning and grammatical comprehension. WFDs in addition were more likely to occur in situations with high processing demands. A variety of intervention strategies were identified. These findings are discussed in relation to current practice and the authors understanding of the condition.