The article is forthcoming in <A href="http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6VFD-4PNM471-3&_user=499884&_coverDate=09%2F15%2F2007&_alid=770199288&_rdoc=1&_fmt=high&_orig=search&_cdi=6008&_sort=d&_docanchor=&view=c&_ct=1&_acct=C000024499&_version=1&_urlVersion=0&_userid=499884&md5=8492c97b78d795d5deec06a94b66422d">Labour Economics</A>.<p><P>