Differences in the representation of diversity in higher education, emphasising the gender gap in some areas, are issues addressed from different research domains. Socially, gender roles have been constructed and are also related to professions. In this context, the Social Cognitive Career Theory explores the possible causes of segregation. This segregation is evident in Europe and Spain, as indicated by the European Institute for Gender Equality. This paper describes the design and validation process of an instrument to find out what opinions university students have about higher education studies in science, technology, engineering and mathematics (STEM), according to gender. After drafting the questionnaire, it was piloted in a non-experimental quantitative design in Spain. Subsequently, a validity and reliability study was applied to validate the items and construct their dimensionality. The process was implemented using Reliability Analysis and Exploratory Factor Analysis. Also, the dimensionality consists of five scales: Gender Ideology, Perception and Self-perception, Expectations about Science, Attitudes and Interests. Based on the results, it is concluded that the opinion about STEM studies is conditioned by personal elements, such as motivations, educational background and family and social influences, such as people who judged their decision, were their references or studied STEM programs. Finally, it is essential to pay socio-educational attention to the modulating components of decisions about which higher education studies to pursue. Awareness of the factors involved in the decision helps the educational community to establish mechanisms to prevent horizontal gender segregation. The instrument designed, validated and presented in this study provides a glimpse of possible causes for the gender gap in STEM higher education.