Research Note: Twin studies and their value for physiotherapy research

Twin studies have been designed and implemented since the late 19th century. Although relatively new and underused in physiotherapy research, they have made a scientific impact on understanding the possible extent of genetic causes behind human traits. The initial applications of twin studies to health and disease were mainly aimed at elucidating the influences of nature versus nurture. More recently, a variety of modern and simple, yet sophisticated, twin research designs have been developed and applied to unravel causes of and cures for diseases and to make significant contributions to the understanding of how psychological and biological traits are formed. Put simply, the availability of data from a family member, in this context a sibling co-twin who shares some or all genetic material and other familial factors with the relevant individual, offers immense possibilities for understanding genetic and other familial influences, controlling for familial confounding, by applying statistical and modelling approaches that are not possible with a non-twin design. Essentially, twin studies can double or amplify the research horizon and provide novel scientific insights into the health of twins and nontwins. This Research Note offers an overview of multiple twin designs and their rationales and potential for use in health research. It presents twin designs that are useful for disentangling the contributions of genetic and environmental influences on health and disease, with a focus on the latter, recognising the relevance of modifiable environmental factors for physiotherapy research. It discusses the use of twin studies to identify risk factors for diseases commonly managed by physiotherapists, through powerful adjustments for genetic and familial influences on outcomes and exposures of interest. The emerging use of twins in randomised controlled trials as a promising approach for researchers and clinicians interested in a cost-effective and efficient way to test the efficacy of interventions in physiotherapy is also discussed. When discussing twin research designs, their applications and advantages are presented and possible limitations and challenges in data analysis and aspects of study setup, such as statistical power and recruitment, are addressed. It finishes by describing how registries of twins can make twin research possible and accessible to researchers globally, and the impact of international twin registries networks.