In Rational Human and Total Brain, the relationship between the human brain and the concept of rationality in the economic sense has been established. One of the major fields of the social science Economics is human behaviour. This study makes a contribution in understanding the decisions taken when making economic decisions. This study was prepared based on the assumption that the Herrman Brain Dominance Instrument (HBDI) and the Economic Rationality Scale (IRS) developed by the researcher would be sufficient to measure rationality.