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What effect does a company's earnings have on the price of its stock?
Your autograph analogy seems relevant to me. But it is not just speculation. In the long run, investing in stocks is like investing in the economy. In the long run, the economy is expected to grow , hence stock prices are expected to go up. Now in theory: the price of any financial instrument is equal to the net present value today of all the future cash flows from the instrument. So if company's earnings improve, shareholders hope that the earnings will trickle down to them either in form of dividends or in form of capital gain. So they buy the stock, creating demand for it. I can try to explain more if this did not make any sense. :)