Barrett is an evolutionary biologist and a worldwide authority on the ecology and genetics of plant reproduction.

Barrett provided the first experimental evidence for the purging of deleterious genes following inbreeding in plants.

Barrett’s research group at the University of Toronto focuses on understanding the mechanisms responsible for the evolution of plant mating strategies, and he has edited several leading books in the field.

[8] In 2006 the Canadian Botanical Association awarded him the George Lawson Medal for lifetime achievement in botany.

[8] In April 2020 Spencer Barrett was elected an International Member (Foreign Associate) of the National Academy of Sciences.