The study that Jeff comments on is dubious. The 'double hump' he discusses is nonsense. The study evidences it from test results where a group of people scored

The study that Jeff comments on is dubious. The 'double hump' he discusses is nonsense. The study evidences it from test results where a group of people scored nearly-full marks and another group of people scored a lot lower, i.e. the inevitable result of any test where each question is testing the same skill.