Psychologists suspect that often results from growing up in below-average intellectual environment.

The given blank can be filled with cultural-familial intellectual disability.

Explanation:

The cultural-familial or socio-cultural intellectual disability signifies towards a state of mild mental impedance, which is considered to be the outcome of environmental elements, generally associated with certain kinds of psychological demerits, impoverished situations, dearth of prenatal care, lack of intellectual stimulation, and poor diet.  

The mentioned condition is generally suspected by the psychologists to have resulted mainly due to growing up in a below-average intellectual environment.  

1. b.

2. d.

3. a.

5. d.

6. c.

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