In the UK zero-hour contracts are controversial. British business leaders have supported them, stating that they provide a flexible labour market. They may suit some people such as retirees and students who want occasional earnings and are able to be entirely flexible about when they work. It has been reported that 60% of people on zero-hour contracts are happy with the hours they work. Trade union groups and others have raised concerns about the possibility of exploitation and the use of such contracts by management as a tool to reward or reprimand employees for any reason, meaningful or trivial. They also raise concerns about how workers can adequately assert their employment rights or maintain decent employment relations.