Behavioural effects of caffeine: the specificity hypothesis
This thesis argues that caffeine use offered a survival advantage to our ancestors and that moderate use continues to offer modern humans benefits. Caffeine ingestion, through the blocking of adenosine receptors, elicits broad elements of the mammalian threat response, specifically from the ‘flight or fight’ and ‘tend and befriend’ repertoires of behaviour: in effect, caffeine hijacks elements of the stress response. If the effects of caffeine had been discovered recently, rather than being available to Homo sapiens since Neolithic hunter gatherer times, it is likely that caffeine would be considered a ‘smart’ drug. More caffeine is being ingested today than ever previously recorded. Caffeine use is found across all age groups, all socio-economic strata, most ethnic groups, and is being used increasingly by the medical and pharmaceutical industries and by the armed forces. Yet despite this wide usage and a substantial body of research literature, there is at present no clear pattern or plausible model for the way caffeine achieves its effects. There is much contradiction in the literature and ambiguity as to why caffeine use should improve performance on some tasks, impair it on others and have no effect on other tasks, for some but not all of the time. The present work argues, through an examination of the specificity of caffeine’s operation, that these effects are not arbitrary but elicited by the nature of the tasks, in particular that caffeine ingestion affects those processes and behaviours which improve the probability of survival under perceived threat or stress. This is argued through the perspective of evolutionary psychology and relies theoretically on Polyvagal Theory. The argument generates testable hypotheses and empirical support for the thesis is garnered from nine experiments on card-sorting, verbal and numerical processing, local and global categorization, field dependence-independence, the Stroop task, tests of visuo-spatial ability, and from a correlational study of caffeine use and personality traits. It is concluded that moderate caffeine use in healthy adults promotes behaviours likely to be adaptive under perceived threat or stress. Limitations of both theory and empirical work and are discussed, together with potential practical applications and suggestions for further work.