A growing body of research has examined the relationship between family socioeconomic status (SES) and educational outcomes. Meta-analyses of raw correlations generally indicate moderate associations, typically between 0.12 and 0.3 for academic achievement and around 0.18 to 0.4 for educational attainment. Causal inference studies, aimed at capturing the true effects of SES, report much weaker associations, usually around 0.1 or less. Despite the importance of these causal estimates, few studies have systematically reviewed evidence from causal research. To address this gap, we conducted a systematic review of studies on the causal effect of SES on educational achievement and attainment. A total of 24 causal inference studies published between 1990 and 2023 were reviewed. The findings contribute to the literature and theory in several ways. First, the meta-analysis revealed a small and non-significant effect of SES on academic achievement (Cohen's d = 0.03) and a small but statistically significant effect on educational attainment (d = 0.08). Second, moderator analyses indicated that parental education exerted a stronger influence on educational attainment than that of family income. Moreover, the absence of significant differences in SES effects between developed and developing countries, as well as across various causal inference research designs (i.e., sample size, model specification, and methodologies), calls into question the assumed context-dependent nature of SES influence. Overall, the findings challenge SES-centered theories, showing that the causal impact of family SES on educational outcomes is much smaller than generally believed, and suggest that universal mechanisms may underlie the SES-education relationship.