Twin research affords an opportunity to study the interplay of heredity and environment. For this reason, twin research is of special interest in the study of physiology of exercise. Genetically, identical twins are the same individual in duplicate because they are the product of a single fertilized egg which splits in half to form two individuals. Non-identical twins are different individuals who through chance were born together. They originate from two different eggs.

In non-identical twins, physique may differ significantly because each partner possesses his own hereditary endowment (fig. 1). By contrast, identical twins represent two editions, so to speak, of the same body type (fig. 2). The degree of correspondence of morphological body features in identical, as against non-identical, twins has been statistically assessed by Newman, Freeman, and Holzinger (Twins, A Study of Heredity and Environment, Chicago, 1937).