The model is intended to show the interplay among a wide array of physical and behavioral aspects (some of which are discussed in this review) and the aging process. The arrows are bidirectional to indicate that the influence can potentially arise from these factors on the aging process, or vice versa. For example, genetic factors could influence how an individual ages, and aging can enhance the effects of genes on specific behaviors. There are other factors that could be included here, such as risk factors for vascular disease or dementia, but this incomplete list gives a sense of how complex the study of aging is, and how difficult it would be to comprehensively assess these variables in a single experiment.