In any democracy, the power rests with the people, but collectively, so that each person has just a little bit and can use this small piece of power to influence the election of the leader of their group. Therefore the question of who can vote is the most important question in a democratic society. Selecting your voters means selecting the people who will have the power to lead, whether it's just the homeowners' association president or a leader for the whole country. Here are some considerations that are taken into account when the question of who can vote is answered.
The most important thing about selecting a voting pool is that the people in the pool should have an interest in the outcome of the election. For example, if your company was holding a vote on which chemical supplier to buy from, they would not consult the employees of the hair salon next door. These hairdressers know nothing about lawn treatment chemicals and have no stake in whether one company is chosen over another. Only that company's employees do. Therefore most democratic voting pools are simply the people whom the leader's decisions will affect.
Does that mean everyone living in a certain country gets to vote in the national election and have a say in who becomes their leader? No. The simple reason for this is that not everyone present is capable of intelligently choosing a leader. Take children, for example. A four year old does not understand issues like compulsory military service and interest rates. Therefore most elections require the voters to be eighteen (or the local age of majority) in order to vote unless it's an election for a student leader in a school or youth club.
Does this mean that all adults in a given country are permitted to vote for the national leader? Again the answer is no. In many cases, organizations guard their values by excluding people that are likely to have interests contrary to the general good of the electoral pool. For this reason, Canada for example, wouldn't let a contractor on loan from another company vote in their company elections and a country won't let a citizen of another country who is visiting or has recently moved vote in a national election unless he or she changes allegiances.
And finally, there are some situations in which the question of who can vote comes down to the question of who owns property. In a situation where a condo association is deciding whether to put their leftover fees into an savings account, only people who own units would be able to vote, not people who are renting. This also sometimes happens in city elections, which are restricted to taxpaying property owners.
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