The bride's oxytocin level shot up by 28% after the vows, "and for each of the other people tested, the increase in oxytocin was in direct proportion to the likely intensity of emotional engagement in the event." Bride's mother: up 24%. Groom's father: up 19%. The groom: up only 13%. Why? It turns out that testosterone interferes with the release of oxytocin—and Mr. Zak measured a 100% spike in the groom's testosterone level immediately after the ceremony.
The guests are affected as well:
It looks as though humans have devised this ritual that induces oxytocin release in a way that bonds people to the wedding party. It provides this social support system, so that this couple presumably can be successful at reproducing.
In a long and worthwhile interivew, Zak expands on the interaction between testosterone and oxytocin:
I already mentioned that stress hormones can inhibit oxytocin, but the favored hormone of half the human race, testosterone, is a potent oxytocin inhibitor. So in the experiments where we administer testosterone to men, we find that they become more selfish and more entitled. So: Who are the most selfish and entitled people on the planet? Teenage boys, which you and I used to be—and we can attest to that. But at the same time, we find that high-testosterone men are much more likely to spend their own resources to enforce social norms of sharing. That is, if given a chance to punish, it’s these high-testosterone individuals who will enforce the rules and punish others for not cooperating. So we have this sort of yin and yang of morality right inside our own beings. We have oxytocin that makes us care about other people, makes us feel empathy—it’s hard to hurt people you feel empathy for—and then we have testosterone, which lets us enforce the rules.