I'm not deeply versed on the issue. I do know that the intention of the immunity is to protect the sanctity of marriage - the idea being that one's confidences in one's spouse are similar to one's confidences in a priest or doctor. However, the exact form of immunity is statutory, albeit based on common law originally. Also, keep in mind that spousal immunity (so far as I know) predates the concept of "no fault" divorce. Meaning that the testimony of one spouse against another had completely different implications vis a vis the marriage.
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Here's a link to a blog discussing a recent MA case on spousal immunity/forfeiture:
http://lawprofessors.typepad.com/evidenceprof/2010/09/804b06-com-v-szerlong-ne2d-2010-wl-3530019mass2010.html
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I think, after reading the case linked there, the equity argument is going to be that since spousal immunity historically exists to protect an existing marriage, one cannot "take advantage" of that - i.e., marry with the
intent to invoke spousal immunity in a pending matter. That makes sense to me. Or to limit it further, perhaps one shouldn't be allowed to invoke if one marries after an indictment. Or further, after if one marries trial has commenced. Whatever the practical rule is, there seems to be a reasonable limitation of the privilege to be found. The question is
when do we allow a spouse not to testify against their spouse? (This isn't strictly a confrontation clause/hearsay issue, given that immunity has been granted.)
Right now, if MA law holds, one cannot invoke if one marries with the intent to invoke - which is a sort of case by case knowledge based standard. (And yes, we'd need an evidentiary hearing.)
To take the absurdist position of unlimited/unreviewable immunity, a defendant could marry (and subsequently divorce) a key witnesses; get a decent pre-nup and one could essentially buy witnesses who would invoke spousal immunity. It looks like gender won't be a barrier to that in the near future. (I'm not making a slippery slope argument, but just pointing out that this is a limited privilege to begin with - for good reason.)
Also, despite immunity, it's not like Jenkins has clean hands in this matter. She's potentially an accessory after the fact to murder.
While there's a Constitutional right to
marry, there's no Constitutional right to invoking spousal privilege. Perhaps AH and SJ didn't get married because they know the privilege wasn't going to work - and it may have created additional civil liability for her. (Just shooting in the dark on that last one.)
Edit - sorry for the ton of edits on this one.