Tithing is a practice that is common in Christianity. The word ‘tithe’ comes from an Old English word meaning ‘tenth’, and refers to a spiritual practice of giving a tenth of one’s income to God. Nowadays, this typically translates into giving a tenth of one’s income (i.e., 10%) to an organization of some kind that one feels ‘called’ (an intuition in Christianity, which supposedly comes from God) to give to. (This doesn’t include taxation, which is relevantly different, for reasons that will become clear.)
Why tithe, though?
On the surface, tithing seems like just giving something up. However, the idea in Christianity is that tithing actually adds to your life. How?
1. Tithing can create a sense of abundance. I.e., it sends a signal to your brain that there is more than enough. This is useful for a couple reasons. The first is that it feels good to have a sense of abundance in one’s life, decreases stress, and so on. Tithing causes us to realize that (at least most of the time) there is in fact an abundance of this sort. Of course, we wouldn’t want this if it were a delusion. This leads to the second reason, that, whatever the psychological mechanisms, when we consistently feel a sense of abundance, that tends to create abundance of various kinds.
2. Tithing also creates a good concept associated to money or material goods. I.e., we lessen our attachment to money, which is probably a good thing, as money isn’t the basis of excellence. This idea is captured in Christianity in the First Commandment (or what you could call the First Sensible Suggestion), to put God first place (“though shalt have no other gods before me”), i.e., don’t value things like money above things that tend to be more important and fundamental to success.
3. Tithing can help out a cause you believe in. This also moves one outside oneself, which tends to be a good thing psychologically to an extent.
Why a tenth (10%)? I think the important part from an individual perspective is that it’s a number that seems significant, and so creates a sense of abundance (and so on). (The psychological difference between 9% and 10% in a base-10 counting system (such as we have) is also significant.)
Reasons 2 and 3 make some sense of the idea attributed to Jesus of Nazareth, that those who give publicly already have gotten their reward. If you tithe without most people in society knowing, you lessen the motivation of doing it so that it will make you look good, and so increase the chances of creating a good association towards money (putting God first place), and of getting oneself outside of oneself (while giving to show off tends to do the reverse).
These 3 things, so the idea goes, can also increase the chances of alignment or connection with God, where being in such a state is a good thing. (Also see here.)