Theory: Economists use the apt term “cheap talk” to refer to mere pledges or promises that are not backed up by credible threats. Generally speaking, a pledge, promise, or vow to do x is worthless when it is not backed up by a serious threat. (Stated in probabilistic terms, a credible threat makes compliance with a pledge more likely.) Some threats are formal in nature, like bringing a lawsuit. Others are more informal, like taking hostages or spreading negative gossip. Such informal threats might even be more credible and effective than legal ones, assuming the promisors in these cases actually care about the hostages or their reputations.
Example: Now, what are we to make of the 2015 Republican presidential pledge (pictured below), which now appears to have become a dead letter? (By the way, we recently wrote up a series of blog posts on the morality of promises, using the Republican pledge as a case study. The first installment of our series is available here.) Should such a pledge be legally enforceable? Either way, maybe political parties could require their candidates to actually pledge some real collateral (like their houses or other personal assets) next time they are asked to take a “solemn pledge.”
prior probability 
It’s interesting that the existence of a pledge document, with blank lines to fill in names, dates and signatures, gives the pledge more “weight” in our culture — as if those blanks on the piece of paper almost demand to be filled in. Setting the paper aside after it has been handed to you is akin to breaking the pledge! An analogous situation (not sure why this came to me) is when you are sitting in a church pew during a service, but you do not belong to that faith, and the communion tray is passed to you, and you just smile and hand it on to the next person in the row. If only that tray had not been passed to me, everything would have been cool!