Brown rats are often used as model organisms. Genetic research is usually done with mice. Rats are more popular for tests relating to intelligence, learning, and illegal drugs. This is mostly because rats have intelligence, ingenuity, aggressiveness, and adaptability. Their psychology seems to be very similar to human psychology. Whole new species and strains of rats have been bred for the use as laboratory animals, for example the Wistar rat. Much of the genome of "Rattus norvegicus" has been sequenced.