Using Conserved Sequences and Desire Paths to Design RPGs
Here are two concepts, one from the world of biology and one from the world of landscaping, that I think are useful when thinking about games.
A conserved sequence is a sequence of DNA that is unusually similar across even distantly related species. Why would a particular sequence stay the same despite natural selection and genetic drift? The theory for most conserved sequences is that they are genes for something so critical that almost any mutation in that sequence is lethal. You can apply the same reasoning to RPGs – look at a mechanic, and the less it changes across decades and between different evolutionary branches of RPGs, the more difficult it will be to run a game without. The level of conservation is a spectrum; on one end are mechanics innovated for a single game and never copied, while on the other end are things that linger through the ages. One highly conserved rule I’ve noticed is that, in games with combat, characters can generally do 1-2 interesting things on their turn in combat; rarely if ever will you find a game where a character can do 4 interesting things in a row before the players pass the dice. Rolling initiative and going in order is surprisingly conserved, despite all the interesting alternatives I’ve seen. Critical successes tend to be conserved, as well. In PBtA games, the use of marking experience or +1 forward instead of permanent stat modifiers or other rewards are highly but not universally conserved. In d100 games, doubles for crits took a while to catch on, but once it did, it quickly spread between systems as a new best practice (I suppose that's less of a conserved sequence and more of a broad comparison of a game mechanic to a heritable trait). When designing a new game or hacking a system, it can be worth looking at other mechanically similar systems, seeing which rules are conserved between them, and giving them particular attention before modifying or discarding them. A conserved rule is not necessarily a good rule or a rule that can’t be changed, but it’s usually a rule worth thinking about borrowing.
Desire paths is a term for the tendency of people and animals to step off the sidewalks and cross landscaped areas in the same place every time, leading to a visible path in the dirt and vegetation worn in place by many footsteps. Some landscapers will try to block desire paths with trees, boulders, benches, and other obstacles. Other landscapers will build basic sidewalks, watch to see where people create desire paths, then pave those paths. Desire paths show up when playtesting a game as well. If playtesters consistently try to do something that’s not in the rules, it can be worth just going ahead and paving the path: building in a rule that allows that action to be completed in some way, whether symbolically or mechanically. This applies to smaller elements of design as well, like using language that playtesters find natural instead of language they stumble over. When developing Fear and Panic, I originally had every source of Fear provide a single Fear. Over playtesting, I found that players really wanted to differentiate sources of Fear by scale, and particularly horrifying revelations would leave them feeling that a single point of Fear was insufficient. I eventually added three tiers of Fear sources to tell them apart. This worked really well with adding an element of chance to Fear, so players could indicate how scared their characters felt by the single fluid motion of how many dice they picked up to roll – a familiar language at the game table.