When presented with attractant (light) together with an amount of repellent (methyl eugenol) that exceeds attractant, Drosophila melanogaster fruit flies are of course repelled, but nine mutants have now been isolated that were not repelled. Although able to respond to attractant alone and to repellent alone, these mutants fail to make a decision when the two are together during the first two months of the study. They are considered defective in a decision-making mechanism. The defect occurs at 34 degrees C but not at room temperature, so these are conditional mutants. Efforts at genetic mapping have been made. Our aim is to discover how decision making gets accomplished and how this results in a behavioral response. We indicate that there is a mechanistic relationship between decision making and the central complex in Drosophila and between decision making and the prefrontal cortex in humans and other vertebrates. Over a period of six months these mutants changed into ones that are attracted when presented with attractant together with what was overpowering repellent before. Nearly full attraction was achieved at fifteen to thirty days. With attractant alone these mutants were attracted like the original parent and with repellents alone they were repelled like the original parent. The mutants have been genetically mapped.