While Bessel’s correction for the sample variance is well known and quoted abundantly in statistics’ texts, a detailed treatment of why the correction is needed at all, does not appear to be prominently mentioned, other than a cursory statement to the effect that the estimator is biased without the correction. In this article, we present a mathematical treatment of the ‘uncorrected’ sample variance and explain why it is a biased estimator of the true variance of a population. This will be of interest to readers who are studying or have studied statistics but whom cannot find the real reason for Bessel’s correction. The reader is assumed to have some prior knowledge of statistics and probability, particularly in relation to random variables and the mathematical expectation operator.