Lincoln (2008) found no support for the specialization hypothesis among full-time employed workers. One study found that among identical twins with one married and the other single, average wage increased 26%. Some studies have suggested this premium is pronounced in the working lives of men after becoming fathers. The "fatherhood premium" is the increase in pay specifically after men becoming fathers. Fathers can expect their salaries to be boosted by 4 to 7% beyond that of their childless male counterparts. The fatherhood premium varies by race, as white father receive larger dividends than do fathers of color. Some studies have suggested this premium is greater for men with children while others have shown fatherhood to have no effect on wages one way or the other. Boosts to fathers' salaries and decreases in mothers' are the result of two intersecting factors. First, parenthood allows and/or prompts men to invest more time in work, while women are prompted to invest less. Second, employers' beliefs of the productivity and worth of employees are influenced by gender, as fathers are seen as more productive, while mothers are viewed as less committed to work and thus less valuable.