Because human psychology and culture are so complex and so variable, it is very hard to come up with a foolproof system for diagnosing mental illness. The first editions of the DSM were strongly influenced by prevailing psychological theories and were poorly linked to empirical research. Some of the diagnoses were controversial and what we would now consider culturally biased. For example, homosexuality was listed as a mental disorder until 1974. While more recent versions of the DSM have made much greater use of empirical research, there are still critics who suggest the diagnoses lack adequate scientific validity. Because of the inevitable flaws in any classification scheme, however, it is assumed that each version of the DSM will eventually become outdated and will need to be replaced by a newer edition.