Money-Making Speculators Must Stabilize Markets; Only Money-Losing Speculators Can Destabilize
"People who argue that speculation is destabilizing seldom realize that this is largely equivalent to saying that speculators lose money, since speculation can be destabilizing in general only if speculators sell when the currency (or commodity) is low in price and buy when it is high."
~Milton Friedman, Essays in Positive Economics (p. 175)
The chart above illustrates Friedman's point. The blue line above represents commodity (or currency) price movements over time in a market WITHOUT speculators (due to a government ban on futures trading, e.g. onions), and the red line represents that same market WITH the fictional destabilizing speculators (as portrayed by the media and politicians) now being allowed to trade. For speculators to destabilize that market, they would have to make prices more volatile over time, and that is what the red line illustrates - there are greater price swings, and greater price volatility (both up AND down) WITH speculators compared to price movements over time WITHOUT speculators.
But saying that speculators destabilize the market above is also the same thing as saying that destabilizing speculators must BUY when the market prices are at a HIGH point (driving them up even higher) and must SELL when the market prices are at a LOW point (driving them even lower). And as we all know, buying HIGH and selling LOW is a sure way to lose money, and speculators can then only be destabilizing if they are LOSING money, as Friedman points out, and therefore the chart above can NOT represent reality.
Conversely, if and when speculators are making money, they have to be buying low and selling high, which would be the same thing as saying that they are stabilizing markets and reducing price variability. And the more "excessive speculation" and "unfair profiteering" they are accused of, the GREATER the role speculators play in stabilizing prices and markets.
Bottom Line: If speculators are making money, they MUST be stabilizing markets. If speculators are losing money, they MUST be destabilizing markets (fictional chart above). But speculators can NOT make money and destabilize markets at the same time.