Investigating effects of steroid hormones on lateralization of brain and behavior

Steroid hormones have been proposed to influence the development of lateralisation of brain and behaviour. We briefly describe the available hypotheses explaining this influence. These are all based on human data. However, experimental testing is almost exclusively limited to other animal models. As a consequence, different research fields investigate the relationship between steroid hormones and lateralisation, all using different techniques and study species. The aim of this chapter is to present an overview of available techniques to study this relationship with an interdisciplinary approach. To this end we describe the basics of hormone secretion and mechanisms of action for androgens, estrogens, progesterone and corticosteroids. Next, general issues related to hormone sampling and hormone assays are discussed. We then present a critical overview of correlational and experimental methods to study the influence of prenatal and postnatal hormones on lateralisation. These methods include hormone measurement in amniotic fluid, saliva, urine, faeces, and blood plasma or serum of fetus, mother and umbilical cord. We also discuss hormone mediated maternal effects, the manipulation of hormone levels in the embryo or mother, hormone treatment in persons with Gender Dysphoria, and the 2D:4D finger length ratio as a proxy for prenatal testosterone exposure. We argue that lateralisation can and should be studied at different levels of organization. Namely, structural and functional brain lateralisation, perception and cognition, lateralized motor output and performance. We present tests for these different levels and argue that keeping these levels apart is important, as well as realizing that lateralisation and the hormonal influence on it may be different at different levels, for different functions and different species. We conclude that the study of hormonal influences on lateralisation of brain and behaviour has not yet exploited the knowledge and wide array of techniques currently available, leaving an interesting research field substantially underexplored.