Which one is less formal: hence, therefore, or thus?

Question

Which of the three is less formal, say, to be used in common speech?

Example:

Cats have a parasite, Toxoplasma gondii. It's harmless to them—but in humans it can affect the heart, liver, eyes. Also the mind. [...] its correlation with suicide."

Answer 1

Your question was about the scale of formality. Of these words I think it goes 1) therefore, 2) thus, 3) hence (from informal to formal).

Hence is often used incorrectly so I would try to avoid it, if at all possible.

You say you are looking for an acceptable informal word, although your example is not something that sounds like "common speech" to me. Perhaps you are making a presentation? I would rearrange it a little bit to avoid the problem.

Cats have a parasite, Toxoplasma gondii. It's harmless to them—but in humans it can affect the heart, liver, eyes, and also* the mind, which is why it has a correlation with with suicide.

• or: as well as the mind. I don't like also at the beginning of a sentence.

Answer 2

In my (non-native speaker's) opinion you can only use hence in your specific example.

The other two would need more elaboration in ascending order of being seen as fact.

• hence its correlation with suicide. - works
• therefore/so it may be/is correlated with suicide. - works
• thus it is correlated with suicide. - works

but have subtle differences in meanings

The correlation is a possibility. This comes across in hence its correlation. Thus its correlation does not work for me. Neither does therefore its correlation nor so its correlation which all sound non-grammatical to me.