I see where you are going with your statement:
> Firstly, a lot of issues don't and can't have any scientific rationale behind them. They're moral judgements. For example, if you want to go looking for scientific evidence for why the death penalty is a terrible idea you won't find any. You'll only find ethical and moral rhetoric about why killing innocent people by mistake is bad, or why the economics of keeping people in prison for decades when you could just kill them is irrational. Science has nothing to say. Science doesn't judge.
I wanted to talk more about your example of the death penalty.
However, if you do look into the death penalty, you'll find:
- in places that lock people up for life instead of killing them, sometimes people are later proved to be innocent and then released.
- in places where the penalty is death, a jury that would've sent a person to life in prison will often choose to release the person instead of kill them, as they aren't 100% certain and the consequence is irreversible.
I think science absolutely has some things to say about this. You could take my two statements as hypotheses and do tests to see if they are true. (This would be a lot like medical tests, but it wouldn't be ethical to have a test group and a control group; you could, however, create a regression model between two similar societies (or the same one at different times) and control for various differences, the chiefest being 'uses death penalty' or not, and answer these questions ("prove"/"disprove" the hypotheses) with some confidence.)
I agree that while you could do a study to see if people are happier and economies fare better in societies with capital punishment ["science"], that the rational and the outcome have nothing to do with science. It is a lot like in the Ted talk "Teaching kids real math with computers"[0] where the speaker explains that math has four steps:
1. Posing the right questions
2. Real world -> math formulation
3. Computation
4. Math formulation -> real world verification
If step 3 was 'Do Science', then it becomes obvious that the other steps lie outside the domain of science, but it does not become obvious that we can't use the tools of science to reason about problems that people disagree on, including moral quandaries.
[0]: https://www.ted.com/talks/conrad_wolfram_teaching_kids_real_...