What makes a worker ant perform one particular task rather than another? From the 1970s to the mid-1980s, researchers emphasized internal factors within individual ants, such as polymorphism, the presence in the nest of workers of different shapes and sizes, each suited to a particular task. Other elements then considered to have primary influence upon an ant's career were its age – it might change tasks as it got older – and its genetics. However, subsequent ant researchers have focused on external prompts for behavior. In advocating this approach, Deborah Gordon cites experiments in which intervention in a colony's makeup perturbed worker activity. By removing workers or otherwise altering the nest conditions, researchers were able to change the tasks performed by individual workers.