Determining cause and effect is tricky.

If I drop a glass on the pavement and it shatters, I know exactly what happened. My clumsiness was to blame.

If I came across some broken glass, however, it’s much harder to piece together how it came to this sorry end.

Congress has determined that unbridled speculation in the oil markets is the cause of a significant chunk of the run-up in oil prices. The Senate has voted to set limits on speculation.

I’ll agree it’s been a factor, but only one of so many factors that influence that barrel of crude as it journeys from under the ground (or sea) to your gas tank.

We live in the most wasteful, gas-guzzling society the world has ever seen. Led by China and India, however, the rest of the world has been giving the United States a run for its oil money. Global demand for crude oil has soared as the world economy has expanded rapidly.

Ask economists why oil costs so much and they usually remind us of the law of supply and demand. But for some reason, oil conspiracy theorists don’t think that answer’s good enough.

There must be more to it. It must be OPEC keeping production down. It must be big oil companies refusing to explore for more oil. It must be financial types who have no intention of ever taking delivery of a tanker of crude.

The trouble with Congress’ picking on speculators is that it’s the equivalent of a witch hunt. Politicians want to burn the bad witches and make our country safe to guzzle again. And if a few good witches get hurt, well …

Except that speculators serve an important purpose. These are risk takers who are buying and selling all the time. They provide liquidity to anyone trading in the market, including those who ultimately might take physical delivery of the oil. Futures prices help businesses plan and adjust.

I’m much more worried about Gulf Coast hurricanes and militant attacks on Nigerian oil pipelines than speculators.

Should Congress tighten the screws too far it could damage a futures market that’s one of the best resources we have left in the oil industry.