Mental defectiveness and Mental retardation

Mental defectiveness noun - Retardation sufficient to fall outside the normal range of intelligence.

Mental retardation and mental defectiveness are semantically related. In some cases you can use "Mental retardation" instead a noun phrase "Mental defectiveness".

Mental retardation noun - Lack of normal development of intellectual capacities.

Mental defectiveness and mental retardation are semantically related. Sometimes you can replace phrase "Mental defectiveness" with "Mental retardation", this phrases are similar.