The contagion effect can also refer to the adoption of voluntary party quotas. Once a party has introduced gender quotas, other parties may do the same. In about half of the Council of Europe member states with party quotas, more than one party has adopted a voluntary quota. This is the case, for example, in Belgium, Germany, Iceland, Ireland, Italy, Luxembourg, the Netherlands, Norway, Poland, Romania, Slovenia, Spain, Sweden, and the United Kingdom. In Belgium, for example, the legal quota served only as a minimum benchmark and some parties increased their voluntary quotas, driven by competition with other parties. In the scientific literature also Norway has been singled out as an example of the contagious effects of gender policy. The contagious effect can be shown in the German case, too: the success of the Green party, having achieved gender parity from the early 1980s on, exerts strong electoral pressure on competing parties, especially the Social Democrats.