What is a common effect of polymerization shrinkage in composite resins?

Polymerization shrinkage in composite resins occurs when the material hardens and contracts, which can create gaps between the resin and the tooth structure. This gap formation is a primary contributor to micro-leakage, as it allows saliva, bacteria, and other oral fluids to penetrate into the space, potentially leading to secondary caries and failure of the restoration.

In contrast, the other options do not accurately reflect the consequences of polymerization shrinkage. Increased strength would generally indicate improved durability of the restoration, which is contrary to the effects of shrinkage. Decreased wear resistance is not a typical outcome of shrinkage; in fact, many composite resins are designed to maintain or even improve wear resistance despite shrinkage. A better marginal seal suggests a perfect fit between the restoration and the tooth, which is compromised due to the gaps created by shrinkage. Hence, micro-leakage is the most relevant effect of polymerization shrinkage in composite resins.