Overstudy does not cause mental retardation.

TThere are quite high as it comes in different types of mental retardation.

literature review on mental retardation with reference

Exposure to lead can also cause mental retardation.

Mental retardation has different levels. There is Mild intellectual disability; moderate intellectual disability; severe intellectual disability; and Profound intellectual disability.

https://www.floridahospital.com/mental-retardation-mr/symptoms-and-signs-mental-retardation