Studies examining the effects of family structure on adolescents' academic achievement have produced mixed findings; however, some evidence exists that adolescents living with two biological parents are more likely than those who do not to have higher levels of achievement. For example, Conger, et al. (1997), found that adolescents living in a household with a separated or divorced mother reported lower 10th grade GPA than adolescents living in households with two biological parents. Again, this is a sample of lower- to middle-class White adolescents in Iowa from largely two-parent households. However, McNeal (1999) found no significant differences in the 10th grade test scores of adolescents who were in single parent households in the 8th grade in comparison to those living in a two-parent household in the 8th grade, even after controlling for important correlates of family structure, such as the adolescent's race, the family's SES, and the level of parental monitoring and involvement in school. Guo (1998), in an analysis of NLSY data, found that family structure was not significantly related to achievement outcomes, after accounting for background factors. Specifically, the proportion of years prior to the assessment in which the adolescents' mother was married, divorced, or never married had no influence on adolescent achievement after including the control for long-term poverty status.