 Revisions to breed standards should "recognise the need to avoid the selection for extreme morphologies that can damage the health and welfare of the dog" and in breeds already with a problem, standards should "encourage the selection for morphologies that will improve the welfare status of the breed".Excessive levels of inbreeding is caused by a small founding population (see founder effect), the deliberate use of inbreeding (including line breeding) to fix desired characteristics, and closed stud books.While inbreeding "can have the beneficial effect of purging some damaging traits", it can fix recessive deleterious traits, result in accidental selection of deleterious traits and also cause reduced fitness (see inbreeding depression).