Facial symmetry is one of a number of traits associated with health, physical attractiveness and beauty of a person or animal. It is also hypothesized as a factor in interpersonal attraction. R.J. Edler cited research supporting the claim that bilateral symmetry is an important indicator of freedom from disease, and worthiness for mating. Facial asymmetries and minor physical anomalies begin to appear early in embryonic development, mainly the first trimester of pregnancy, and can be a sign of instability during this growth. Fluctuating asymmetry (random differences between two sides, as opposed to the deliberate natural asymmetry in some animals) develop throughout the lifespan of the individual and is a sign of the phenotype being subjected to some levels of stress.