Women date up and men date down. In the dating world, there is a clear hierarchy. Like most hierarchies, there are few people at the top while the majority are distributed throughout the middle. That leaves a small minority of people at the very bottom. It's set up this way, because most things naturally follow the 80/20 rule. This rule applies to many things, but as far as dating goes, 20% of the population have what it takes to attract at least 80% of the population.
To make things easier to explain, I'll separate men and women into 2 different hierarchies. I'm doing this because woman generally hold more value than men, so there's really no point in grouping them together. The differences between the two do correlate, though. For example, 80% of men are at least somewhat attracted to 80% of women, but only 20% of women are somewhat attracted to at least 80% of men.
Now, that I've set up a foundation around the dating hierarchy, you'll better understand why women prefer to date up and men choose to date down. At a quick glance, it's obvious to see that most women generally have more options than most men. That plays a huge factor in their dating standards. It is to be said that men have high preferences, but women have high standards. What I mean by that, is most men base their dating standards on what's consistently obtainable, while most women base their standards on what they desire most. BARS.
Those are straight bars of wisdom, but I already know some of y'all don't know what the fuck I'm talking about, so let me simplify it even more. Lets say the average person, on a scale of 1-10, is a 7 when it comes to looks. Let's also say that there is an equal amount of avaliable male 7s as female 7s, and that goes for every ranking 1-10. Logically, you would think all the 7s would end up together, because they're equal, but that's not the case.
Because of the previously stated hierarchy, everyone wants to be with a 10. The problem is, the 10s want to date each other, so there aren't that many 10s to go around. The 7-men realize this and set their standard at consistently dating 7-women, but the 7-women will pass on these guys in the hopes of attracting 10-men. This causes 7-men to lower their standards even more to 5 or 6-women, hence men "dating down." The 7-women go through a similar process when they realize 10-men will only go as low as dating 8-women, but they lower their standards to 8-men, hence women "dating up."
The psychological ramifications of this, are men have become comfortable dating down, because it's more convenient. Even if a man is a 10, he knows it takes less effort to impress a woman that's an 8 than it does a woman that's a 10. This causes an inflated self-esteem in women as well as a false sense of entitlement. Imagine, a 8-woman who gets hit on by a few 10-men. She's going to use those instances to convince herself that she's too good for an 8-man. And that's consequently how women started dating up.
I used attractiveness to measure dating value just as an example, but the 1-10 scale could apply to anything you value in a partner. It could be looks, money, intelligence, game, personality, status; whatever. That's why you'll sometimes see a smart successful woman dating a bum ass street nigga. As far as dating goes, she values his grittiness over her own stability. People will look at their statuses and think she's dating down, but he makes her FEEL like she's dating up because she loves guys with an edge. The guy might value big titties, but this woman has A-cups, so he FEELS like he's dating down. But he doesn't mind, because the woman he's attracted matches the level of effort he's willing to put in. Got it now?