Crusader Kings III – Eugenics 101: Perfecting Offspring Through Incest

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:

A few other important modifiers:

Homosexual, asexual, and bisexual: Homosexual and bisexual have a 5% chance to show up for each character. Asexual has a 1% chance. It does not clearly state what happens with these but at least with asexual pregnancies become rare. The closest modifier I could find seems to suggest that asexual characters have a fertility penalty of 90%

  • I am not sure if homosexuals also have a fertility penalty.
  • If one of the characters is landed and they have no children they both get a 20% fertility bonus (primary spouse only).
  • If characters are closely related they get a 10% fertility penalty unless they are part of a religion with divine marriages.
  • If both characters are unlanded they have a 15% fertility penalty.
  • If characters are considered “in the pool” they have a 75% penalty. This is unlanded wandering individuals that show up as guests in your court.
  • Randomly generated characters have a fertility range of +10% to -20%.

Children limits:

Each couple has a (presumably soft) limit for how many children they will have. The base starting numbers are as follows:

  • Unlanded = 1
  • Baron = 1
  • Count = 2
  • Duke = 3
  • King = 5
  • Emperor = 5

Landed characters then have their limit increased by 2. This is added to the above numbers.

There is also a 50% chance that the limit is lowered by 1.

The player automatically has this increased by 2.

Each secondary spouse or concubine increases the limit by 2.

When the limit is reached it appears to give a 90% fertility penalty.

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.

Strengthen Bloodline Decision

This gives you a +40% chance to pass on good traits (+70% with the legacy perk…aka almost 100% chance to pass on good traits at all times) and a guaranteed chance to gain new traits.

The strengthen bloodline decision requires:

  • You to be the head of the dynasty.
  • To have one trait from the mind, body, and beauty lines.
  • And of those 3 lines at least one must be the max level (herculean, beautiful, or genius).
  • And for you to have no bad congenital traits.


This section is one I am still trying to figure out…I am missing some numbers and need to do some testing.

Since this section is about the dynasty and family lets also cover special traits related to close marriages which are inbreeding and pure blood

Inbreeding: This is about as bad as it gets…you get -5 to all skill, a sizable health penalty, a fertility penalty, and a few other things. It is essentially all bad traits rolled up into one. The chance to get the inbred strait is as follows (final number is a percentage):

Number of common ancestors X corresponding level factor X inbreeding chance modifier.

The number of common ancestors is just that, how many ancestors do the parents have in common. Common ancestors are determined as 4 generations back from the child (parents, grandparents, great grandparents, and great great grandparents).

The definition of common ancestor is ” A common ancestor is an individual that occurs on both sides of the family tree or pedigree; i.e., an individual that contributes genes through both the mother and the father. If just one common ancestor exists, the individual’s parents are related and are inbred. If there are no common ancestors, then they are not inbred” (inbred in this does not mean the trait, this is the actual scientific definition of inbred).

The level factor is based on how many common ancestors there are and it is determined as follows

  • 15 = 0.33
  • 17 = 0.75
  • 20 = 1
  • 22 = 2
  • 23+ = 3

And finally inbreeding chance modifier. I could not find what this number is so I have no idea how to calculate the chance to receive inbred.

keep in mind the inbred trait itself and shared genetic material (the % numbers above) are not the same thing. I will refine this section more when the numbers can be tested to see how far I am off.


The more shared relatives (4 generations deep), the higher the chance of inbreeding.

To get the pure blood trait (rare) you need to share at least 20 common ancestors with 23 being optimal. Cousins (when counting 4 generations deep) share 18 relatives so to get this trait you need to have cousins marry together for multiple generations at the least. I would not really actively chase this trait too hard because it only has a 2.2% chance to show up when all conditions are met. Your best bet is to continually marry cousins together which you will be doing anyway to gather up all the traits into one line. The legacy perk that lowers your chance of getting bad traits is essential for this as bad traits like inbred seem to block pure blood from being acquired!

It will happen over time and then you can breed it into your dynasty like any other trait. You could also, assuming you can find it, bring someone into the family who already has the trait.

Examples and Strategies

So in this section I will be visualizing some examples as well as laying out some strategies on how to actually get traits into your bloodline and how to keep them going and collecting.

Some things to keep in mind for this section is that I will not be including any modifiers unless explicitly stated and I will not be including the detailed inheritance mechanics from the last section, just the basic inheritance model from the top.

First lets visualize basic inheritance…nothing fancy this is mostly what you will see the AI doing and what you will do if you are more worried about an alliance or something other than keeping traits.

You may also do this to bring in new blood to help prevent inbreeding too much but in such a case you would still want the father in this scenario to have any trait as opposed to none:

Now lets look at an “optimal” setup starting from scratch to bring in and hold a trait in your blood line:

In this case, even the children who do not get the trait will still be useful for breeding and keeping the trait in the family, just make sure you pair them with someone who has the trait!

Ok so how do we get a new trait into the line? This is where things get messy and a “weak link” is introduced in the form of a lower chance to inherit everything…this weakness lasts for at least 1 generation beyond the first as well. This is why the legacy Blood perk line is super important to getting and keeping all these traits. In the beginning it looks like the above image except only slightly altered. Despite being only “slightly altered” the complexity is much higher without proper tools or methods to track such things. In this picture B means beautiful and F means fecund since those are the two traits I recommend starting with.

The TL;DR version of the chart is that you have to inbreed to get multiple traits onto a single character or characters and you must reinforce those traits through inbreeding or marrying into the missing trait.


  1. If both parents have the same trait (quick and quick for example) there is a 50% chance to improve to the next level up.
  2. Always marry into the highest trait (if you have quick marry an intelligent or genius person).
  3. If you have the highest trait (genius for example) always marry someone else with the same trait, if not marry someone with the highest available trait in the same group (such as quick or intelligent)
  4. Start your dynasty by focusing on fecund or beautiful. Beautiful is preferred as it’s easier to keep because it has multiple levels to it.
  5. If two parents have the same trait and a child is born with no traits do not panic. Marry them to someone with the same trait as the parents. They have a high chance to have children with the trait as their trait is simply “inactive”
  6. Getting the level 1 legacy perk in the “Blood” line is absolutely essential…otherwise you will be fighting RNG the whole time. The rest of the traits in the line are not as essential but still helpful.
  7. To bring in a second trait to your lineage make sure everyone at least 2-3 generations deep has the first trait and don’t attempt it without getting the above mentioned blood legacy perk. When you try and introduce a new trait it creates a weak link that risks losing the trait you have and not gaining the traits you want. This is why fecund and beautiful should be your first traits you integrate because it’s a number game and they give you a lot more babies to increase your chances of heirs with the right traits.
  8. Use councilors and knights to bring traits into your court and help have babies with good traits (that are not of your dynasty) so when you have children you can marry them to one of the children of your knights or councilors without too much inbreeding. Just make sure these “pet” dynasties you create have jobs or the head of the family will leave with all their children.
  9. Using your on family, create 2 lines of traits at the start if possible. If you start with no kids, spend the first generation focusing on one trait (marry someone with beautiful and have all your first generation kids marry someone with beautiful). Any “duds” from this lot that don’t have or produce children with the same trait should be used to bring a new trait into the family.
  10. If you start the game with children, make one branch focus on beautiful and the other focus on fecund. Down the road these lineages will intermarry so you can have dynasty members with both fecund and beautiful.
  11. When trying to inherit multiple traits you will most likely be inbreeding as the AI focuses on alliances and prestige, not traits. They do focus on total skill points though which means genius and intelligent traits are going to be slightly more likely among AI since those give blanket stat boosts. Once again though the AI doesn’t care about the actual specific traits. This means you need to be mindful and bring “new blood” into the dynasty from time to time to prevent a horrid inbred spiral. This is where your “pet” dynasties from knights and councilors come in really really handy.
  12. Every ~10 – 15 years you need to use the character finder and make sure everyone in your court is married and making babies with good traits. The easiest way to manage this is to time it with events such as pilgrimages (once every 15 years) or feasts/hunts. If your knights and courtiers have made children with good traits make sure they have jobs so they don’t leave…either as knights or courtiers (don’t forget about court physician!).
  13. Pinning characters to track is super helpful. Always pin the most notable offspring from your dynasty or pet dynasties to ensure you can keep tabs on and marry off all important trait bearers as soon and as frequent as possible and also to keep track of their children.
  14. Religious that allow multiple spouses are superior to religions that allow concubines. Concubines are selected from your own court while secondary spouses can be picked as normal. This means it will be very hard to use concubines for trait breeding while a character with 4 spouses can easily have them all have the same traits or different traits that they want to have in their children down the road.