What would be the limit on demand if something was free? If the product or service was free, there would be no limit on demand. For public property or large pools of assets and there is no requirement to pay or contribute before taking, the supply then quickly disappears. The principle of contribution states that those who contribute to a common pool will limit their demand.

The principle of contribution has several benefits. First, if one must pay to receive, there is a natural limit on demand to those who can pay. Secondly, if one must contribute in order to take, the number of times someone can take is limited. The act of having to contribute also inserts a pause, forcing those who would use resources to consider whether or not they must take it and its value. This reduces waste. The requirement that one contribute to the common store before taking from it also acts as a source of replenishment. 

The principle of contribution helps resolve the tragedy of the commons. In the tragedy of the commons, the common land is owned by everyone. In the end, it is overused by everyone and becomes unusable. By requiring everyone to contribute, such as helping maintain the commons or put money into a communal storehouse, the personal investment in the commons rises. This decreases abuse of the commons and increases its value to those who have contributed to it.

Withdrawals may be based on need or rank or privilege. However, a pay to play as outlined by the principle of contribution ensures that all gain a right to the common property and help sustain it. The principle of contribution makes all contributors stakeholders. These stakeholders are more likely to report theft or hoarding because this will impact themselves in the long run. They will protect the communal store because they have a right to its contents. Thus the principle of contribution helps sustain the sustaining resources on which the group relies.

The converse occurs with something for nothing. In socialized medicine, when care is free, people are more willing to rush to the doctor for colds and check ups that amount to social visits for the elderly. This increases demand for medical services without changing its supply. In charitable giving by food donation programs around the world, those areas that received endless free food found themselves requiring more and more aid. Why? Because once food became free, they no longer paid farmers who grew it. Farmers stopped growing food because they could not afford to give it away. Lands without farmers have no food, and thus the need for food hand outs only deepened. Mosquito net given to the poor in Africa had little impact on malaria. Why? Because free mosquito nets were turned into bridal veils and fishing nets, curtains and carrying nets. What was free was freely taken, if rarely for the intended purpose. When people were charged a few cents for the mosquito nets, however, the rate of malaria decreased in those areas. Those who could not previously afford nets for several days of pay could afford them for less than an hour of work. But because they had to pay for it, they used it for their own purpose. The mosquito nets were thus put to their intended use by those in need, and the few cents paid by millions helped buy more nets to be sold at a discount in stricken areas.

Human nature has shown that something for nothing quickly leads to nothing for everyone. The principle of contribution is the only solution to the tragedy of the commons.

This is why the principle of contribution is used across industries from homeless shelters that charge $1 per night to "free" Habitat for Humanity homes that require sweat equity. Something for nothing - i.e. free - leads to nothing at all. If one must contribute something, anything, then it becomes valued and retains its value.