Occupational gender segregation is one of the most persistent gender inequalities in the labor market and one contributing factor is the lack of women in STEM fields. Possible explanations for this pattern are the characteristics of the fields of study (creative-associative versus analytical-systematic thinking style required, the number of mathematics courses, competitive atmosphere), and the characteristics of the job for which the field of study prepares for (risky transition to the labour market, social versus technical skills, income, prestige and family-friendly working conditions). However, previous research was not able to tell which factor really contributes to the gender segregation as the characteristics of the fields are highly correlated and cannot be disentangled with survey data. For this reason, I conducted a choice experiment with high school students two years before they start university. The results show that women and men have different preferences except for prestige and risk aversion and that they differ the most in their self-image (thinking style: 20 percentage points difference; technical versus people-oriented skills: 25 percentage points difference).