Comment by pron

> while it's easy to do harm (lead exposure) it's all but impossible to do any good.

That's just a meaningless statement no different from "while it's easy to subtract negative numbers, it's all but impossible to add positive numbers."

> or non-systematic (results from non-linear interaction of environment and genes).

Non-linear interaction does not mean non-systematic. Computer programs are fully deterministic (and therefore "systematic") while being non-linear (and therefore generally unpredictable). It is true to say that when things are non-linear it's hard to tell with certainty what effect some policy will have, but given that most human systems are non-linear, this is true for just about everything.