Workers’ compensation and personal injury claims often become embroiled in debates about the cause of the clinical presentation. When the primary claim involves an extremity, affected individuals sometimes report subsequent symptoms in the contralateral, previously “normal” limb and often attribute its onset to overuse while favoring the initially involved extremity; such an overuse hypothesis seems plausible, and perhaps intuitively obvious, to some. The concept that favoring one upper limb can result in injury to or illness in the other is not based on scientific evidence and instead is an unsupportable myth. Determining relationships between risk factors or exposures and medical conditions is a complex process that is outlined in the Guides to the Evaluation of Disease and Injury Causation (Causation). A search on PubMed and MEDLARS using the key phrases “opposite uninjured arm” and “uninjured arm” returned only six relevant articles, and the authors report that claims of serious or persisting painful syndromes in the arm or hand opposite the injured one are seldom adequately supported by clinical evidence. Similarly, the literature does not support “favoring” as a reasonable cause for development of symptoms in the contralateral shoulder or elbow. Epidemiological studies can provide general information regarding risk; this information must be filtered by specific steps to assess causal association for a disorder and determine if the injury is work related. Findings then can be applied to the specific individual.