The advent of the AIDS epidemic in the 1980s spurred states to reevaluate their sex education policies and, in some cases, expand their requirements. Most states require that public schools teach some form of sex or STI/HIV education. Most states, including some that do not mandate the instruction itself, also place requirements on how abstinence or contraception should be handled when included in a school district's curriculum. This guidance is heavily weighted toward stressing abstinence; in contrast, while many states allow or require that contraception be covered, none requires that it be stressed. Further affecting whether students receive instruction on sex or STIs/HIV are parental consent requirements or the more frequent "opt-out" clauses, which allow parents to remove students from instruction the parents find objectionable.