Significance
Complex learning and memory are believed to require the weakening or elimination of synapses in the brain, a process mediated by adhesion molecules, which maintain synapse strength and stability. In the present study, we examine in vivo the effects of stabilization of β-catenin, an intracellular protein that is a component of the cadherin adhesion complex. We find that stabilization of β-catenin in the brain prevents normal activity-dependent downscaling of synapse strength, resulting in a striking impairment in cognitive flexibility. These results demonstrate that β-catenin plays an important role in learning and memory and that aberrant increases in synaptic adhesion can have detrimental effects on cognitive function.