Human hair color account for all possible shades of brown, blond, or red (for example, platinum blond versus dark blond/light brown), nor does it explain why hair color sometimes darkens as a person ages. Several gene pairs control the light versus dark hair color in a cumulative effect. A person's genotype for a multifactorial trait can interact with the environment to produce varying phenotypes (see quantitative trait locus). Natural hair color can be brown, blond, black, red, or white; all natural hair colors including grey, white and lightest blond, are shades of brown. The Fischer–Saller scale, named after Eugen Fischer and , is
Spann-Fischer Codependency Scale the Department of Human Development and Family Studies at Texas Tech University. In a series of studies, Fischer, Spann, and Crawford (1991); Fischer and Crawford (1992); and Fischer, Wampler, Lyness, and Thomas (1992) defined co-dependency as a dysfunctional pattern of relating to others with "an extreme focus outside oneself, lack of open expression of feelings, and attempts to derive a sense of purpose [exclusively] through relationships" with others. The Spann-Fischer Scale has enjoyed good test-retest reliability ( > .80), and acceptable internal consistency (.62 < < .92), across studies. Spann-Fischer scores have been associated with membership in Codependents Anonymous, gender,