Acyclic graphs illustrating the multivariable regression (MVR) design for (a) a set of independent ASD-related variants Gi (ASD-MVR) and (b) a set of independent ADHD-related variants Gj (ADHD-MVR). For both MVR models, increaser marker alleles (Gi or Gj) were aligned to ASD (ASD), ADHD (ADHD) and EA (EA) GWAS SNP estimates, capturing genetic association at the single-variant level. Using a bidirectional MVR framework (ASD-MVR; ADHD-MVR), the aggregate association effect with EA across all variants was simultaneously estimated for ASD risk (ASD; #ASD) and ADHD risk (ADHD; *ADHD), including a regression intercept (0*; 0#). *Estimation of polygenic risk effects using alleles that were selected to increase liability for ASD, but are shared, by position, with ADHD risk. #Estimation of polygenic risk effects using alleles that were selected to increase liability for ADHD, but are shared, by position, with ASD risk. (c) ASD-MVR (ASD; *ADHD) and ADHD-MVR (ADHD; #ASD) effects as change in years of schooling per increase in log odds of ASD or ADHD liability. Multivariate inverse-variance-weighted regression estimates and corresponding 95% confidence intervals (bars) are shown. Individual effect estimates, standard errors and corresponding P-values (t-statistic, two-sided test) are provided in Supplementary Table 3. All tests passed the multiple-testing threshold of P < 0.0023. Gi was selected from ASD(iPSYCH, woADHD) and Gj from ADHD(iPSYCH), both at Pthr < 0.0015 and Pthr < 0.05. ASD (ASD), ADHD (ADHD) and EA (EA) SNP estimates were extracted from ASD(iPSYCH, woADHD; N = 32,985), ADHD(iPSYCH; N = 37,076) and EA(SSGAC; N = 766,345) GWAS statistics respectively. 3D scatter plot of (d) ASD-MVR (Gi; Pthr < 0.0015) and (e) ADHD-MVR (Gj; Pthr < 0.0015), as shown in (c). The regression plane reflects the estimated MVR effects for ASD-MVR (ASD; *ADHD) and ADHD-MVR (ADHD; #ASD), respectively. Source data are provided as a Source data file. ADHD Attention-Deficit/Hyperactivity Disorder, ASD Autism Spectrum Disorder, EA educational attainment, MVR multivariable regression, Pthr, P-value threshold, SNP single-nucleotide polymorphism.