What did Jean Berko Gleason’s Wug Test show about children’s grammar skills?

Jean Berko Gleason’s Wug Test showed that children as young as four can correctly form plurals, past tense forms, possessives, and present participles for words they have never heard before. For example, children could turn the invented verb “zib” into “zibbing” and call someone who performs the action “a zibber.” These results supported Gleason’s conclusion that children have a predisposition to learn and use grammar rules.