His greatest interest was in twins. He thought that if he could discover what caused twin birth, he could use the knowledge to increase the birthrate of Germans. Some twins were deliberately infected with the same disease and then given different treatments to see which was more effective. On occasion twins’ blood supplies were switched or they were given transfusions of different blood types. Mengele forced twins to have sex with other twins to see if still more twins could be produced. Pairs of twins were killed simultaneously so that their bodies could be dissected and examined for any anatomical differences.
Despite the fact that the doctors produced few usable results, the medical experiments were considered very valuable by the Nazi leadership in Berlin. The SS doctors drew high praise for their work. An evaluation of Mengele boasted "He, as an anthropologist, has most zealously used his little off time duty to educate himself further and, utilizing the scientific material at his disposal...has made a valuable contribution in his work to anthropological science."