What does it mean to say that some people have higher psychometric intelligence than others? Does the ability to answer questions correctly and quickly on a mental test reflect real-world intelligence? ‘Brainy’ asks whether there are measurable aspects of brains and brain functions that differ between people and that also relate to psychometric intelligence differences. There must be some biological quality of the human brain that makes some people cleverer than others, such as size, electrical activity levels, reaction time, or the efficiency of visual processing. Our knowledge of the brain's workings is still incomplete.