There is growing evidence that science capital (science-related forms of social and cultural capital) and family habitus (dispositions for science) influence STEM career decisions by youth. This study presents reliability and validity evidence for a survey of factors that influence career aspirations in science. Psychometric properties of the "NextGen Scientist Survey" were evaluated with 889 youth in grades 6-8. An exploratory factor analysis (EFA) found four factors (Science Expectancy Value, Science Experiences, Future Science Task Value, and Family Science Achievement Values). Using confirmatory factor analysis (CFA), four separate factor models were tested. The CFA affirmed that the four-factor solution extracted during the EFA was the best-fitting model. The analyses also found acceptable internal consistency for each of the four factors. The results validate the use of the "NextGen Scientist Survey" for measuring science capital for middle school youth.