(n.) The state of being backward.

arrested development backwardness blithering idiocy cretinism half-wittedness idiocy idiotism imbecility infantilism insanity mental defectiveness mental deficiency mental handicap mental retardation mongolianism mongolism mongoloid idiocy moronism moronity profound idiocy retardation retardment simple-wittedness simplemindedness simpleness simplicity subnormality