One parameter that impacts the robustness and reliability of forensic DNA analyses is the amount of template DNA used in the polymerase chain reaction (PCR). With short tandem repeat (STR) typing, low copy number (LCN) DNA samples can present exaggerated stochastic effects during the PCR that result in heterozygote peak height imbalance, allele drop out, and increased stutter. Despite these effects, there has been little progress toward decreasing the formation of stutter products and heterozygote peak imbalance effects during PCR. In the current study, the effects of the PCR additives were assessed by evaluating STR typing results. Of the four additives, the only positive effects were observed with betaine treatment. Betaine, at a final concentration of 1.25 mol/L, was found to improve the robustness of the amplification, specifically by decreasing stutter in a dual locus system. In contrast, the addition of 1.25 mol/L betaine to commercial STR amplification kits did not affect stutter ratios; however, the addition of betaine did lead to increased yield of PCR products in all commercial kits tested. The results indicate that betaine can improve amplification efficiency of LCN DNA samples. (Publisher abstract modified)