Right... but humans have a much richer way of signaling than most animals. My point is that some humans will find ways to signal commitment without sinking three months of their salary into a worthless rock. Signalling theory is an explanation for observed behavior, not a guarantee that such behavior will persist if the organisms in question develop alternative strategies.
I like to think humans are pretty good at developing alternative strategies.
Besides, if you get divorced, you don't get your down payment back anyway.
Sure, as I said in another comment, diamonds could probably be replaced. But it would probably be by something just as useless.
Or maybe if you can find a way to mimic the effect, but make it useful, you have your hands on a very valuable startup idea.
I guess it would be something like helping friends move, which is considered a good signal for true friendship. I don't know (not an expert on this), maybe if you find a way to "help your spouse move" for the equivalent worth of 3 months of your salary, you are good.
Why don't you get your down payment back if you get divorced?
I like to think humans are pretty good at developing alternative strategies.
Besides, if you get divorced, you don't get your down payment back anyway.