Doctor Copper

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Market lingo for the base metal that is reputed to have a Ph.D. in economics because of its ability to predict turning points in the global economy. Because of copper's widespread applications in most sectors of the economy - from homes and factories, to electronics and power generation and transmission - demand for copper is often viewed as a reliable leading indicator of economic health. This demand is reflected in the market price of copper. Generally, rising copper prices suggest strong copper demand and hence a growing global economy, while declining copper prices may indicate sluggish demand and an imminent economic slowdown.

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Investopedia explains 'Doctor Copper'

Investors are cautioned that Doctor Copper is not infallible, and should not be relied upon as the only indicator of economic health. For example, a temporary shortage of copper may lead to rising prices even as the global economy is slowing down; conversely, a copper glut may cause lower prices despite robust economic growth.