MINNEAPOLIS – Divorce can have a severe impact on children — hurting their performance in math, increasing their anxiety and short-circuiting their social skills — but not necessarily as soon as families and family therapists expect it.

While research has shown that children struggle with divorce, a study released Wednesday by the University of Wisconsin-Madison is one of the first to determine just when those struggles emerge.

Reviewing the test scores and social well-being of 142 grade-school children with divorced parents, researcher Hyun Sik Kim was surprised to find no significant decline in the months leading up to their parents’ divorces.

It was only during the divorces themselves that struggles emerged.

And two years later, the children remained behind their classmates from two-parent homes. “They do not recover fully,” Kim said, “and they do not catch up with other children from intact families.”

Kim’s findings were not a surprise to counselors and attorneys. For children, there’s a difference “between parents just fighting, and fighting and divorcing,” said Phyl Bean, a Bloomington, Minn., lawyer.

“While the parents are still together, they’ve got predictability and some sort of stability, even though it’s bad,” she said. “Once the divorce has started, then they’ve really got to face that issue.”