Where is the line. Mental retardation has two primary criteria. An IQ score that is 2 standard deviations below the mean and difficulty with adaptive functioning. On the weschler intelligence scales, that means an IQ score below 70 (with a mean of 100 and a standard deviation of 15), and a documented difficulty with functioning in daily life activities. The vineland scale of adapative functioning is a test used.
Ability/achievement. Intellectual disability (mr) is classically defined as having an overall iq of <70 with low scores in social/adaptive functioning in 3/10 areas (ability to function independently in society). A learning disaabilty is an unexpected difficulty learning in a child with an average to above average iq. Classically, achievement scores are 15 points or greater less than iq in reading, writing or math.