PITTSBURGH, July 22 (UPI) -- A U.S. study determined gains in reading and math by children who attended full-day kindergarten did not last past the third grade, researchers say.

In fact, the researchers found the academic skills of children in part-day kindergarten grew faster in grade 2 through 5 than those of children in full-day kindergarten. The fade-out, the researchers said, can be explained, in part, by the fact that the children in part-day kindergarten were less poor and had more stimulating home environments than those in full-day programs

"The results of this study suggest that the shift from part-day to full-day kindergarten programs occurring across the United States may have positive implications for students' learning trajectories in the short run," study leader Elizabeth Votruba-Drzal of the University of Pittsburgh said in a statement.

"They also highlight that characteristics of children and their families play noteworthy roles in why the full-day advantages fade relatively quickly."

The study, published in Child Development, was based on a nationally representative group of kindergarteners made up of 13,776 children from the Early Childhood Longitudinal Study -- Kindergarten Class of 1998 to 1999.