Stephen Jay Gould, the biologist and author, once joked that were he an identical twin raised separately from his brother they could ‘hire ourselves out to a host of social scientists and practically name our fee’. In order to monetise Gould’s fantasy, one would want a form of twinship that could operate according to evidential, experimental, somatic and circumstantial ideals. And Gould admits that he and his brother would need to be viewed as ‘the only really adequate natural experiment for separating genetic from environmental effects in humans’. This chapter seeks to interrogate the evidential and experimental circumstances that may underpin the comic quips that guide modern biology. In human genetics, twins are used as experimental bodies that are made to matter in particular ways and for particular people; they become newly ‘animate’ for being enrolled into scientific research. Raised in cultures assumed to be alike or dissimilar, isolated by researchers for being valuable in the measured disentanglement of assembled molecular agents (which are sometimes distinguished from an assemblage referred to as an ‘environment’), twins achieve a status of experimental significance not just for what they do but also for what they are taken to be.
