Provided is method of strengthening keratinous fibers that includes contacting the fibers with a composition that includes a poly(vinylamine-vinylformamide) copolymer and a carrier, wherein the treated fibers exhibit an improvement in strength. Also provided are methods for improving strength of fibers that have been damaged, e.g., as a result of having been subjected to chemical and/or thermal treatment processes that diminish fiber strength, by contacting the fibers with the composition.