Phrenology is a defunct medical practice that gained popularity in Victorian medicine. The idea is that the shape and lumps on your skull are indicative of character and personality traits. It was first developed by Franz Joseph Gall, a German Physician in 1796 and it involved taking measurements of the head and identifying the lumps and indentations Gall claimed related to 'brain organs'. There were 27 individual organs that were said to determine personality and the use of measurement gave the practice an air of scientific practice and made it seem plausible. Edinburgh was a centre for phrenology research at the time and was home to the Edinburgh Phrenological Society, Their last meeting was in 1870 at their premises on Chambers Street.
Phrenology has a dark history of use in eugenics. It was thought that different skull shapes from different races indicated intellectual capacity and the practice was popular during a time of exploration and the expansion of the Empire. Some of those on expeditions were followers of phrenology or were paid by them to collect skulls and not everyone died of natural causes for these purposes. Skulls of people from large distances away and unusual places were prized by their collectors. The skulls were studied to identify differences to understand the reason why there were perceived intellectual differences between races. Followers of phrenology believed that there was a hierarchy of intellectual capacity, with - of course - white people at the top.
Many of the skulls still in use in medical schools throughout the world are from this time of skull collection for Phrenological “research”. There are skulls on display in the museum in Edinburgh that were undoubtedly collected in a dubious manner. Phrenology was used to justify the superiority of the Tutsi people over the Hutus in Rwanda which resulted in the genocide in the country. Terrible people will use shitty ideas to justify horrific actions. Phrenology has thankfully lost favour and is no longer given any credibility, but there are still people out there who try to compare the intellectual capacity of races, they just no longer use this junk science to justify it.