Human physical appearance is the outward phenotype or look of human beings. There are infinite variations in human phenotypes, though society reduces the variability to distinct categories. Physical appearance of humans, in particular those attributes which are regarded as important for physical attractiveness, are believed by anthropologists to significantly affect the development of personality and social relations. Humans are acutely sensitive to their physical appearance, some theorize for reasons of evolution. Some differences in human appearance are genetic, others are the result of age, lifestyle or disease, and many are the result of personal adornment.