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Crusader Kings III - Eugenics 101: Perfecting Offspring Through Incest

Written by Crimson Bolt   /   Sep 7, 2020    

Not sure how people are making babies with every possible good trait? I will show you the math and method that will ensure the best methods to pass on as many traits as possible and collect traits on well crafted characters!

Guide to Perfecting Offspring

Basic Mechanics

For anyone coming from CK2 you will likely be surprised by the much more complex system...luckily this is a good thing for us because it makes it more likely to reliably pass on traits instead of just hoping (with a fairly low chance) that our kids get the traits we want them to get!

Fertility and Pregnancy

Before we get into traits though lets cover how exactly babies are made in CK3

How pregnancy is calculated (checked every month because they know how to keep that sex life going in CK3):

And fertility is calculated (before modifiers) as:

Concubines/Consorts and secondary spouses have half the fertility for all fertility checks and after every child a woman will lose 5% fertility.

Pregnancy progression chance assuming marriage at age 16 and no modifiers and no births:

Trait Inheritance: It's Really Simple!

So how does the inheritance system work? The easiest way to lay it out is like this:

As a bonus, if both parents have the same level of trait such as level 1 of the intellect trait (quick) then there is a 50% chance it will be upgraded to the next level (Intelligent). That being said, for reasons you will see in the next section it is ALWAYS better to have one parent with the top trait even if the other parent only has the first or second level. You should only pair 2 characters with the same level trait if they both have the second highest trait. Only pair a level 1 trait (such as quick) with a level 3 trait (such as genius) if you literally have no other choice or you have the level 1 Blood legacy perk (detailed further below).

Detailed trait inheritance: I Was Lying When I Said It Was Simple

Trait Inheritance Levels:

  • Active - this means the person has the trait.
  • Inactive - this means the person does not have the trait, but it still has a strong presence in their genes.
  • None - This means they get no trace of the trait in their genes.

Keep in mind that these trait levels have nothing to do with the grandparents as seen in the chart above. Only the parents matter in this situation. The above chart simply shows the chance of children getting a trait (actively) based on their grandparents. I do not know if grandparents can give children inactive traits.

The following is essentially a second layer to the trait system. The above mentioned chart is used to determine if they get the trait and the second layer determines what level of the perk they get. This is why even if you have 2 geniuses with the legacy perk having a kid (110% chance to pass it on) you can end up with kids who are not geniuses or do not even have the perk at all.

How are these states determined?

When a baby is born:

First the chances of acquiring the trait as an active trait is checked as a function of the parents:

  • Active - Active: 80%
  • Active - Inactive: 50%
  • Active - None: 25%
  • Inactive - Inactive: 10%
  • Inactive - None: 2%


If that check fails the chances of inheriting inactively is calculated:

  • Active - Active = 150%
  • Active - Inactive = 100%
  • Active - None = 75%
  • Inactive - Inactive = 50%
  • Inactive - None = 25%

The above system applies to all traits that do not have multiple levels, It starts to get a little more tricky to follow for the traits that do:

General logic for trait with tiers inheritance

Start from highest trait

Calculate active chance. Lower tiers ( including inactive ) count as inactive of the tier. Each level down beyond the first multiplies chance by a define.

If active, use it; done.

Otherwise, go down a tier, if inactive pretend 1 more parent has that tier active, otherwise 1 more parent has it inactive. Goto 2.

When trying to inherit a tiered genetic trait, and the other person has one of lower tier, we pretend that means they've got the higher tier one recessively, and reduce the chance of inheritance by 20% per tier further down than 1 the other parent's trait is When going down a tier, add 40%

Legacy Traits: The Most Important Part to Make This Smooth Like Butter

So that seems pretty straightforward (it's not really, at least for leveled traits) but lets talk about the most important part of ensuring the traits you want get passed on without letting chance mess it up to much: Blood legacy perks!

If you check out the legacy perks you will notice the "Blood" line of perks and oooh boy are they powerful so lets take a look


Level 1

+30% Chance to inherit good congenital traits.
+30% Chance of new good congenital traits.

Level 2

  • +30% Chance to reinforce congenital traits.

Level 3

  • -30% Chance to inherit bad congenital traits.
  • -30% Chance of new bad congenital traits.

Level 4

Select a congenital trait to become more common from:

  • Comely, Quick, Hale, Fecund, Albino, Giant, Dwarf, Scaly.

Level 5

  • +5 Life Expectancy.

Unfortunately some of it is a little vague but lets go over the important parts:

Level 1

This is the big one...if you are not super focused on purely breeding (but even if you are) and you want to grab other legacies at least grab this level before anything else. Adding a 30% chance to pass traits on or have them pop up on their own at random is huge.

It is a little vague and states "good traits" which I assume the game classifies as Comely, Quick, and Hale as they have green backgrounds and their levels are labeled X_good_1/2/3 in the files.

Obviously traits like albino and giant are mostly good but have some penalties so I am not sure if they are classified as good, bad, or neutral. The same applies to fecund...it has a green background but I have no idea if it is classified as a "good" trait or just a neutral one.

Level 2

This level is arguably just as important...for reasons we will see later the level of trait inherited (quick, intelligent, or genius for example) is not always as straightforward or simple as a binary check to see if they get it or not. This perk allows you to salvage a situation where a perk is less than the top level and you can't find someone to marry with the top level available or especially early on in the breeding program to help get started on the right foot even if you can't find the top tier of traits in a spouse and your Petri dish of generational perfection doesn't have the highest trait themselves. Either way it takes more of the RNG out of the whole situation and puts it in the your favor.

Level 3

This is where the benefits start dropping off but this is still an important trait...primarily for AI because while they prioritize good traits they are not near as focused as a player and are willing to allow some bad traits to sneak into the family line. Alternatively for you it allows you to be a tiny bit more flexible when you really need new blood in the system...perhaps there is a genius that you need to reinforce and ensure your own geniuses...but he has the ugly trait...with this perk the risk is worth it because the chance of getting that trait is near or at 0%

Level 4

I am not really sure how or where this applies...I assume it's on new traits that randomly appear but I am sure it increases the chance of the trait getting passed on.

**I am totally speculating and pulling it out of my ♥♥♥ but I would assume this sets the trait to one step above it's current state for the purposes of inheritance. We will get to those states in the next section. If it works like I am totally assuming with no proof or evidence than it is actually really good for giant, albino, fecund, scaly, or dwarf as these work on a ternary system (only three states) while intelligence, beauty, and herculean have 3 states for each level and are easier to keep going in the bloodline.**

If that's not how it works....then it's pretty much trash because we are relying on skillfully applied genetics...not a low chance for random traits to make their way into the system (though the ones mentioned before would still benefit from this at a higher level than the leveled traits)

Level 5

This one is just a nice cherry on top...doesn't really help much for the purposes of what we are doing. That being said it allows men to spread their seed longer and when coupled with fecund gives you a +10 life expectancy. That being said, life expectancy extends the range that females can have children according to the tooltip in game. So coupled with fecund the cutoff for pregnancy should be 55 instead of 45 for women in your dynasty.


Written by Crimson Bolt.