It's a matter of motivating the player.

• The player has an achievable goal to work towards


• The player sees how far away from the goal they are


• The player sees how playing the game visibly progresses them towards the goal

Variable ratio rewards can also be a great motivator, but so is an "honest pay for honest work" game mechanic where the reward after each action is guaranteed. The player never feels like doing something was pointless, because there is always at least a minimum reward in form of experience points.

Many MMORPGs recognized this and made the levelup progress bars much more accessible than classical single player RPGs. They are often very prominent, always visible UI elements. I have seen MMORPGs which made the EXP progress bar span across the whole width of the screen. Some even make it a two-tiered bar with one bar showing the progression between 0% and 100% and a smaller bar below showing the progress to the next full percent. That way players can still see visible progress when they get into the lategame phase where it takes several days to level up.

Using experience and gold as a reliable reward and item drops as a variable reward has proven to be a practicable and effective combination. So it basically became a standard convention of the RPG genre. There is of course no reason why you shouldn't try to do it the other way around in your game. I could see it work quite well. But keep in mind that you are in experimental territory here.

In more "skinnerboxy" games you see a lot of such numerical progression systems. Progress to next level, progress to levelup individual skills, progress to completion of quest, etc.. The player should always find something to work towards.

One secret of addicting game design is to layer many different reward systems which stimulate the player's reward neurons in as many different ways as possible:

• Short-term, mid-term and long-term rewards


• Constant, slightly variable and you-just-won-the-lottery variable rewards.

Think about what would happen if level up happened randomly. You have a party of three players. At the beginning, all are the same level (for simplicity, 1). After playing around one levels and two don't. Not a huge deal normally. Since the one leveled, the others will level next and catch up. But under your system, the one who just leveled may level again before the others do. Over time, you end up with one person who was really unlucky and is level 5. Another is level 7. And the lucky one is level 11.

How does the system balance a party with a level 5 and a level 11? The level 5 player will die if the encounter is set up for a level 11, and the level 11 will be totally bored if the encounter is set up for level 5. And if they compromise and set up for level 8, they have both problems at the same time. The level 5 is constantly dying while the level 11 is constantly bored.

Now, perhaps you play solo. So this doesn't matter to you. But if the game is designed to include team play, it has to handle this issue. And they can't really have different behavior for this in solo versus team, as the same characters can participate in both. Even if your game is solo only, players will expect it to work like this.

Another problem is when the luck is going against you. You're stuck at level 5. You can't advance because you're waiting to level. You get tired of the game and quit, because it's a constant grindfest as you wait to level.

A third problem is when you level early. There's a certain amount of level 5 content. But you leveled and can do level 6 content. Do you stay and do the now easy level 5 content? Or do you switch to level 6 content? If you switch, you're essentially skipping part of the game. Your enjoyment is reduced because you didn't get the full content. And if you do the level 5 content anyway, you get bored as the encounters are not challenging to a level 6.

It can get worse. While you're doing the extra level 5 content, you may level again. Now you're a level 7 doing level 5 content.

This problem is reduced with loot, as it doesn't progress the same. You find a good sword. How lucky. Then you find a great sword. But having the good sword doesn't make the great sword any more powerful. Having a level up event on top of another level up event is more powerful than a single event. So the second amplifies the first. But with the swords, each drop is isolated.

All this isn't to say that you can't have random variation in the experience rewards. But with team play, you want all the members of the team to get the same experience result. And in solo, you have to find ways to match the difficulty to the player's capability. If you have both, then you need to be consistent between them.

The short answer: psychology.

The long answer:

We humans are driven by the expectation of a reward, just like a dog. Knowing how much experience you have reached and how much is still needed for reaching the next level gives you a nice feedback that what you do is good or bad for evolving your character.

Edit: also, not knowing how long it will take for levelling up is like downloading a big file without a progress bar. It may take the same 10 minutes, but without a progress bar it will feel like 10 hours

There is actually a quite interesting GDC talk that has been given on this topic by someone who worked on the reward structures for large sections of World of Warcraft and Diablo 3:

https://www.youtube.com/watch?v=urijgWXLYck

He talked at length about how players experience randomized versus deterministic rewards and how both reward types work together to provide a player with a sense of progression while also encouraging them to continue playing. I also recommend a related video from someone who played World of Warcraft for a long time, a video that gives his own comments on the video and clarifies a number of things from the perspective of the player:

https://www.youtube.com/watch?v=OjQAnJYyXf4&t=0s

I recommend watching both videos if you have time, because they provide a good overview of the challenges with reward structures and how he solved deterministic versus random rewards.

He found that in order to keep players playing, they need to feel like they improved meaningfully every time they played, even if they only played a short while and only got a small reward. That's the role that EXP delivers: you can play a short period and still feel like you did something worthwhile with your time. He also found that if everything was deterministic, the player dynamic would change and they would view the game more and more as a really grindy system that was very predictable in pacing and felt like a deliberate slowdown of the game. This was especially noticeable during Icecrown Citadel and Firelands, when Emblems of Frost and Dragonwrath respectively changed a large and impactful part of progression to a mostly deterministic path, where you could predict when you'd get the next step in your gearing process months in advance and guilds planned around them.

He had exactly the reverse problem in Diablo 3: the loot players got was too random and often not usable for their class, so players would go days or weeks without any upgrades because the few legendaries they'd get were meant for one of the other 4 classes. The result is that players would stop playing the game for their OWN loot and instead played to get as much loot as possible to sell on the ingame Auction House so they could then buy their own gear with the proceedings. It turned the game entirely deterministic and players were quitting the game in droves because the core part of the game felt unrewarding. They additionally also had the problem where their intended max level deterministic reward, Paragon levels, would eventually cap out and players would no longer see reason to continue playing for them.

That problem was solved through removing the AH, redesigning Paragon so there was no level cap, and rebalancing player loot drop rates so they would get far more legendary rewards and far more rewards that were suited for their class. They then found that their loot had pretty much become deterministic, which they fixed by adding Ancient items, which essentially were Legendary items with more stats on them. They also added a way to more or less target specific items, but without losing the random chances.

TL;DR

• People are impatient.


• More randomness tends to make games more difficult.

As you have pointed out, it's been scientifically proven that many species find variable ratio setups 'irresistible'. This is because they appeal to a very primitive part of the brain, and assuming that the individual judges risk/reward ratios correctly, it's actually a really important survival trait. Because it's such a low-level response, it's really hard for most people to not follow up on it, and therefore it does indeed make a good draw for a lot of players.

The problem is, this impacts behavior in ways people might not expect. People don't like to wait for results to be visible, they want them now, which in turn means that for many people, no visible progress towards a reward equates to zero progress towards said reward, which then means that most (sane) people will stop trying to get that reward after a while. This is actually the same reason that a 'clicker' or whistle is used when training animals, it helps fill-in the space between the desired action and the tangible reward, making it easier for the animal to equate the action and the reward mentally.

As a result of this, people tend to get irritated when they make no visible progress after spending multiple hours working on something (not just in games, but also in real life). The exact point at which this starts to become an issue varies from person to person, so most games err on the conservative side here and give you easy access to exact information about how far you've progressed.

Some people go even further than this though, and want to know exactly what they need to do to to achieve something, and as a result you can find information on exact amounts of XP you get from any given action in a number of the more populate RPG's out there with a simple search of the internet.

All that psychology is useful info for explaining this, but there's an even more important, and much simpler, line of reasoning that explains it, leveling is one area where more randomness makes a game really hard, and if you're not going to make it random, you might as well just show players the numbers because they're going to figure them out themselves through experimentation anyway.

Imagine trying to level on a bell curve. You have to get some minimum amount of experience to possibly get the next level, are guaranteed to get it after getting some higher amount of experience, and most people level about halfway between those two numbers. This has all kinds of issues:

• In a single-player RPG, it makes the game much less consistent. This can technically increase replay value for veteran players, but it can make the game hellishly difficult on a person's first play-through (or insanely easy, in which case they may not end up liking the game as much either).


• In a drop-in co-op RPG (like Borderlands), it makes it very difficult for people to actually work together as a team. Part of the reason such games work at all is that they provide reasonable guarantees that a group of people who start the game at the same time and only play together will have roughly equal power levels. Without that guarantee, the whole format falls apart (because anybody who gets lucky a couple of times will constantly steal the spotlight).


• In an MMORPG, it just makes things miserable in general. Imagine playing WoW or Warframe but not knowing when you'll be high enough level to join your clan/guild on their next big raid. It's just as bad for sensible clan/guild leaders, because they can't realistically organize long-term coherent groups of people who are about the same level to progress together.


• In any type of RPG, it makes coherent story progression nearly impossible (oh, I finished this series of story quests, but I'm not quite high enough level yet to be able to even attempt the next set of story quests, guess I have to go grind enemies from the last quest for a couple of days instead of getting further in the story).

It also, somewhat ironically, would make the developers life much more difficult. This approach would require a lot of complexity in the code that ahndles leveling, and complex code is more difficult to maintain, more difficult to debug, and more likely to have bugs. Even systems which have you send XP directly on power ups instead of leveling are simper than this.