My guess is that those who say no to this question live in religious societies, while those who say yes, live in secular ones. And both would be right too!
But in your view; why does theism (belief in deity/deities) lead to immoral (unconventional ethics) behaviour?
Humbly,
Hermit
First off, some clarification, since I see you defined "immoral" as "unconventional ethics": when I use the terms "moral" and "immoral," I'm talking about welfare. Immoral acts are acts that are contrary to the welfare of thinking beings. Immoral behaviour can be very much conventional behaviour in some societies and settings.
A few ways that theism can result in immoral behaviour:
- it can discourage reflection. The believer projects their values onto God, then infers "God's will" based on that. Because it's "God's will," it's followed with less questioning than would be applied to the believer's own judgment normally.
- it can come as a package deal with positions on factual matters that aren't actually correct that influence the morality of a decision. For instance, is it moral or immoral to leave someone to suffer needlessly? If their suffering will earn them overwhelming reward in Heaven, then it can be seen as moral. If Heaven isn't real, then it's probably immoral.
- theism can also come as a package deal with anything from deference to clergy to full-blown theocracy. These can serve as shields for immoral acts: for instance, accusing a priest of abuse can come with a major social cost, which reduces the likelihood of reporting abuse that does happen.