She was into S&M and Bible Studies*

"IQs of children ages 2 to 4 who were not spanked were 5 points higher four years later than the IQs of those who were spanked. The IQs of children ages 5 to 9 years old who were not spanked were 2.8 points higher four years later than the IQs of children the same age who were spanked.
'How often parents spanked made a difference. The more spanking the, the slower the development of the child's mental ability. But even small amounts of spanking made a difference,' Straus says."

Found here.

Why? The authors suggest that spanking may cause post-traumatic stress syndrome that makes learning more difficult.

Spanking leads to other problems, such as sexual dysfunction, general aggressiveness,

What's a parent to do?

Unlike spanking, however, verbal punishment alone didn't affect either children's aggression or their cognitive development. But interestingly, when verbal punishment was accompanied by emotional support from moms, the children did better on the tests of cognitive ability.

My mother stopped spanking when she came to the realization that "you can't teach a child not to hit by hitting them."

* Win a prize if you get the reference.