Our Mendelian randomization analyses indicate causal effects on PCOS aetiology for higher BMI (odds ratios: 1.90 per +1 s.d., 95% confidence interval: 1.55–2.34, P=2.5 × 10−9), higher insulin resistance (1.11 per +1 s.d., 1.05–1.19, P=6 × 10−4) and lower circulating SHBG concentrations (0.86 per +1 s.d., 0.78–0.93, P=5 × 10−4) (Table 3). This evidence concerns the gene SHBG and Insulin resistance.