The value of a unit of currency will almost always change over time. This happens when the amount of goods and services that the currency can buy changes. For example, while a dollar may buy a certain amount of gold a particular year, the next year it may allow the person to purchase significantly less. When a unit of currency gains value, this is known as deflation. Deflation has a number of different causes.
Monetary Policy
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One of the main factors affecting the value of a particular currency is the actions of the central bank of the country that issues the currency. Most central banks can control the supply of a particular currency by printing money or by moving more money out of circulation. If the central bank takes money out of circulation, then the supply of the money will decrease relative to demand, making it more valuable.
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Bad Economy
A bad economy can lower the demand for goods. This can have several effects. First, companies may be forced to lower their prices to compete. This means that a unit of currency will be able to buy more goods and services than before -- deflation. In addition, this may lead to a decline in wages, which can exacerbate the decline in demand, triggering a deflationary spiral.
More Demand for Currency
Often, currency is owned not just by the people who use the currency to buy goods and services, but by international investors. When more investors are buying up a particular currency, this lowers the supply of the currency relative to the demand. This makes the currency increasingly valuable, as there is less of it available. However, if the investors choose to sell off the currency, the value can fall again.
Increase in the Supply of Goods
If the number of goods available within a particular country rises in relationship to the demand for these goods, then, following the law of supply and demand, the price of these goods will fall. Just as in a bad economy, when demand for goods falls, here an increase in the supply of goods will trigger deflation, too. This can also trigger a deflationary spiral, if the oversupply is not checked.