Forward stutter, or over stutter, one repeat unit length larger than the parent allele (N + 1 stutter), is a relatively rare product of the PCR amplification of STRs used in forensic DNA analysis. We have investigated possible explanatory variables for the occurrence and size of forward stutter for four different autosomal multiplexes. In addition, we have investigated models used to predict the expected heights of forward stutter. For all tetra and penta-nucleotide repeats we can find no correlation between allelic peak height, marker, or longest uninterrupted sequence in the allele. The data fit a gamma distribution with no explanatory variables. For the single trinucleotide repeat present in two of the four multiplexes (D22S1045) forward stutter is much more common and the best explanatory variable appears to be back stutter height. This suggests some fundamental cocausation of high backward and forward stutter for this locus.