Stephen trained in genetics and molecular biology with Professor Kim Kaiser at the University of Glasgow. The aim of his Ph.D. Thesis was to develop a reverse genetics approach, enabling the generation in Drosophila of mutants corresponding to any chosen gene. They developed a technique, termed “Site-selected mutagenesis”. Though the method was developed specifically for the transposon-mutagenesis of Drosophila, it was extended to other organisms and to other mutagenic strategies. Stephen used this technique to ask specific questions about the roles of cellular components in learning and memory in Drosophila. He exploited the method to generate new mutants in the RI regulatory subunit of Protein Kinase A, and established a role for this protein in Pavlovian olfactory conditioning in Drosophila. This prompted his major scientific interest, the role of genes in directing neural development and behaviour. 

In 1994, he was invited by Prof. Jeff Hall at Brandeis University to join his laboratory as a post-doctoral fellow to exploit molecular-genetic and behavioural approaches in the fruit fly to understand how the sexual identity of a nervous system and its behaviours are specified. He worked on a mutant fly gene, nicknamed fruitless, that specifically disrupts male reproductive behaviours. As part of a team of scientists from several universities they isolated the gene that controls sexual behavior in male fruit flies. Their research was the first to pinpoint a single gene (fruitless) that works in the brain to govern nearly all aspects of a complex behaviour in adult animals. At the end of 1999 he returned to the UK as a Senior Lecturer at the University of Glasgow. Stephen moved to Oxford as a Lecturer in Biomedical Sciences in 2009. He is also a Tutorial Fellow in Medicine and Physiology Sciences at Magdalen College.

Stephen uses the fruit fly, Drosophila melanogaster, to study the genetic, developmental, and neural mechanisms that underlie sex-specific behaviours in higher animals. In particular, the elaborate courtship ritual performed by the male fly has provided remarkable insights into how the neural circuitry underlying sexual behaviour, which is largely innate in flies, is built into the nervous system during development, and how this circuitry functions in the adult.  Innate behaviours refer to the actions of an animal that manifest themselves without prior experience, and thus by implication are genetically inherited. Yet how does gene expression control the development and function of the nervous system so that a gene's action influences some discernible aspect of behaviour? Stephen’s group are studying how the Drosophila transcription factor genes fruitless and doublesex act within the complex and highly organized network of transcription factors to orchestrate the developmental events necessary for sex-specific behaviours and physiology, and the broader lessons this can teach us about the mechanisms underlying the development of sex-specific neural circuitry.