We report a method for tuning the domain spacing ( D) of self-assembled block copolymer thin films of poly(styrene- block-methyl methacrylate) (PS- b-PMMA) over a large range of lamellar periods. By modifying the molecular weight distribution (MWD) shape (including both the breadth and skew) of the PS block via temporal control of polymer chain initiation in anionic polymerization, we observe increases of up to 41% in D for polymers with the same overall molecular weight ( M ≈ 125 kg mol) without significantly changing the overall morphology or chemical composition of the final material. In conjunction with our experimental efforts, we have utilized concepts from population statistics and least-squares analysis to develop a model for predicting D based on the first three moments of the MWDs. This statistical model reproduces experimental D values with high fidelity (with mean absolute errors of 1.2 nm or 1.8%) and provides novel physical insight into the individual and collective roles played by the MWD moments in determining this property of interest. This work demonstrates that both MWD breadth and skew have a profound influence over D, thereby providing an experimental and conceptual platform for exploiting MWD shape as a simple and modular handle for fine-tuning D in block copolymer thin films.