Uncommon Ground

Causal inference in ecology – Counterfactuals

Causal inference in ecology – links to the series

Let’s start with a few preliminaries.1

  • A causal factor (“cause” for short) is something that is predictably related to a particular outcome. For example, fertilizing crops generally increases their yield, so fertilizer is a causal factor related to yield. The way I think about it, a causal factor need not always lead to the outcome. It’s enough if it merely increases the probability of the outcome. For example, smoking doesn’t always lead to lung cancer among those who smoke, but it does increase the probability that you will suffer from lung cancer if you smoke.
  • Causes precede effects.2 That’s one reason why teleology is problematic. A teleological explanation explains the current state of things as a result of, i.e., as caused by, something in the future, namely a purpose.3
  • Effects may have multiple causes. The world, or at least the world of biology, is a complicated place. Regardless of what phenomenon you’re studying, there are likely to be several (or many) causal factors that influence.

The last point is one of the most important ones for purposes of this series. When we are investigating a phenomenon,4 we’re trying to discern which of several plausible causal factors plays a role and, possibly, the relative “importance” of those causal factors.5

To make this concrete, let’s suppose that we’re trying to determine whether application of nitrogen fertilizer increases the yield of corn. That means we have to determine whether adding nitrogen and adding nitrogen alone increases corn yield. Why the emphasis on “adding nitrogen alone”? Suppose that we added nitrogen to a corn field by adding manure. Then increases in the amount of applied nitrogen are associated with increases in the amount of a host of other substances. If yields increased, we’d know that adding manure increases yield, but not whether it’s because of the nitrogen in manure or something else. Why does this matter?

From very early on in our education we’re taught that “correlation is not the same as causation.” We want to distinguish cases where A causes B from cases where A is merely correlated with B. Yet, as David Hume pointed out long ago, experience6 alone can only show us that A and B actually occur together, not that they must occur together (link). One way of distinguishing cause from correlation is that causes support counterfactual statements. They provide us with a reason to believe statements like “If we had applied nitrogen to the field, the corn yield would have increased” even if we never applied nitrogen to the field at all. The only reason I can see that we could believe such a statement is if we had already determined that adding nitrogen and adding nitrogen alone increases corn yield.7

How do we determine that? Randomized controlled experiments are the most widely known approach, and they are typically regarded as the gold standard against which all other means of inference are compared. That’s where we’ll pick up in the next installment.

  1. As I warned in the introduction to the series, I am not an expert in causal inference. The terminology I use is likely both to be imprecise and to be somewhat different from the terminology experts use.
  2. Philosophers have argued about whether backward causation is possible, but I’m going to ignore that possibility.
  3. Biologists sometimes use teleological language to explain adaptation, e.g., land animals evolved legs to provide mobility. It is, however, relatively easy (if a bit long-winded) to eliminate the teleological language, because natural selection shows how adaptations arise from differential reproduction and survival (link).
  4. Or at least this is how it is when I’m investigating a phenomenon.
  5. I’ll come back to the idea of identifying the relative importance of causal factors in a future post.
  6. Or experiment.
  7. If there are any philosophers reading this, you’ll recognize that this account is horribly sketchy and amounts to little more than proof by vigorous assertion. If you’re so inclined, I invite you to flesh out more complete explanations for readers who are interested.

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