The Ericsson method separates male and female sperm by passing them through a column filled with blood protein, human serum albumin. As the sperms enter the human serum albumin, the differences in mass between the X and Y chromosomes manifest as the lighter male sperms push deeper into the protein than the females dragged down by the weight of the extra "leg" of the X sex chromosome. This tiny difference creates separate layers of concentrated male and female sperm. The layers of gender-selected sperm are of higher concentrations but not pure. This lack of purity explains the 30% chance of gender selection failure of the Ericsson method.