Section 5: Breeding - therabidsquirel/The-Fallout-Shelter-FAQ GitHub Wiki

5.1

Q: Can I control SPECIALs in the children I breed?

A: Yes, but you have limited control. There are only two methods of influencing SPECIAL, one being the chance at getting rare/legendary children with high stat parents (see 5.4), and the other being highest stat inheritance. Children will always have a "primary" stat, one that just starts off higher than any others. It does not change their total SPECIAL, just which is highest with the points they have. With common children for example this will be the stat that starts at 3 while the others have 2 or 1. Children inherit the primary stat from either parent at a 50/50 chance. The primary stat of the parent is whatever their highest stat is at the time of conception (outfits not included).

If a parent has multiple stats tied for highest, the game has a built-in algorithm for determining which stat it considers to be the highest for that dweller. This algorithm is different whether you're on version 1.18 (Android and iOS) or version 1.13 (every other platform).

For 1.18, all I currently know is that strength is inherited when all stats are tied (like max SPECIAL).

For 1.13, this post (Apples_Brother) has the breakdown for every combination of tied stats and what stat wins the tie and is inherited. Of note:

When all stats are tied (like max SPECIAL), intelligence is inherited.
No combination of 5 or 6 tied stats results in endurance being inherited. Only at certain combinations of 4 tied stats or less can it be inherited.
If just 2 stats are tied, then which is inherited follows the hierarchy E>C>P>I>A>L>S, where endurance always wins and strength always loses.
Of the 120 different combinations of tied stats, the following are how many combos result in inheritance for each stat:
- C: 51, E: 27, I: 17, A: 14, L: 7, P: 4, S: 0

5.2

Q: How do "Twins Chance" pets affect breeding?

A: There has been plenty of debate about when you have to equip them, but I currently believe the following. The pet just needs to be equipped on the female dweller as she becomes ready to give birth, the moment the baby icon appears over her head. With multiple pregnant dwellers you want to use the pet on, you're probably best staggering when each of them gets pregnant by a tiny bit, and remembering the order. Then check in just before they're ready and equip the pet on each in the same order as they become ready.

I previously thought the pet had to be equipped on the female dweller both at the moment she's ready to give birth, and also at the moment of conception. From more conversations I've had it seems like just the moment she's ready to give birth is all that's needed.

If you successfully use a Twins Chance pet for a pregnancy you also get a chance at triplets, which is a global 2% regardless of the bonus of the pet. While I previously thought it was impossible to gets twins or triplets without the pet, I've had one reported case on iOS of twins with definitely no pet, so I'm not sure. Maybe it was a glitch, maybe they are possible but rare without the pet. If you know more there's an issue here. Maybe its version-dependent, leading into the next point.

There's a hidden benefit to having twins though which is that their SPECIALs are linked to each other, meaning if you train any stat of either twin, regardless of where the other is they both gain the increase. This includes spending Quantums to speed up training, both twins get the increase but Quantums are only consumed for one of them. You can fully max both of their SPECIALs by just training one of them. Use this to your advantage by having them in different training rooms at the same time, so they train twice as fast, or by putting one to work somewhere while the other does the training for both. Regular levels are not linked, they still have normal level ups independently. Here's the post that drew my attention to it. While it's likely that triplets also have their SPECIAL linked then, because their chance is so low this isn't something I've confirmed yet.

There's two caveats to the previous point though, fully outlined in the issue here. One is that twin SPECIAL linking only works in version 1.13, meaning Android and iOS which have had exclusive updates don't have SPECIAL linking. Two is that even in version 1.13 it doesn't work for some people, my guess being a bug, but I haven't been able to track down a cause or get twins that aren't linked.

This pet can be synergized with a Child SPECIALs pet. Twins Chance needs to be equipped when the baby icon shows up, while Child SPECIALs then needs to be equipped when you tap the baby icon.

5.3

Q: How do "Child SPECIALs" pets affect breeding?

A: Equipping the pet on a female dweller for giving birth (by tapping the baby icon) will add the specified amount to each of the child's SPECIALs. This means if the pet says +3 child SPECIALs, the child gets a total 21 extra SPECIAL points. You can easily swap one pet around between women before tapping their icons.

This pet can be synergized with a Twins Chance pet. Twins Chance needs to be equipped when the baby icon shows up, while Child SPECIALs then needs to be equipped when you tap the baby icon.

5.4

Q: Why was my child born with high SPECIAL?

A: Parents with combined 122+ stat points have a chance to produce rare children. Rare children (like rare dwellers) start with 28 total stat points. Parents with combined 134+ stat points have a chance to produce legendary children. Legendary children (like legendary dwellers) start with 40 total stat points. SPECIAL from outfits are not counted in determining chance for rare/legendary children. See 18.1 for an example screenshot of both. The only advantage they have over common children is a higher starting point for their SPECIAL, which has the biggest benefit in less training time. The chances are relatively low for either (see 5.7), so if you want a bunch you'd be best to have lots of max SPECIAL dwellers go at it.

5.5

Q: I want to increase my population, should I breed or use radio rooms?

A: A lot of people prefer breeding. Radio rooms only give you common dwellers (12 total SPECIAL) at level 1 in normal, level 5 in survival. They come with no equipment in normal and a low damage common weapon in survival. Calling a dweller runs the risk of triggering deathclaws though, see 6.15.

If you take into account what I've said in 5.1 through 5.4, you'll see you can do a lot more with breeding. By combining 5.2 through 5.4 (have max SPECIAL parents, equip Twins Chance pet for the end of the pregnancy, then equip Child SPECIALs pet for birth), it's possible to get lots of very high starting SPECIAL children, saving tonnes of training time. Radio rooms will get the job done, but are comparatively very limited. The only real advantages radio rooms have over breeding is you get dwellers that aren't related to anyone, and you get adult dwellers whenever you call someone and skip waiting for three hours each of pregnancy and childhood. The latter is assuming you've got signals waiting in your radio rooms though, as the signals can take a lot longer than that to generate.

In survival there's the additional consideration of called dwellers being level 5 and having a low damage common weapon. The weapon is nice, free items are always good, but coming at level 5 means their health is not optimal. See 4.1 for the details on how health works. Their health will be calculated as if they hit level 5 with their endurance, which will always be in the 1-3 range. In survival, where health is much more important than normal, those 4 level ups with low endurance don't help. Dwellers with 1-3 endurance get 3-4 HP per level, compared to the 10-11 HP per level with 15-17 endurance. Given how easy it is to just breed and train level 1 dwellers, the health penalty of called dwellers doesn't seem worth it.

5.6

Q: What's the population limit?

A: 200 dwellers, and explorers always count. This is achievable with five level 3 triple living quarters. Be aware though, you can't demolish living quarters if it would drop your capacity beneath your population, and you can't build or upgrade more living quarters if doing so would give you more than 200 capacity. This means if you have 200 dwellers, you're not going to be able to move the living quarters or build new ones without evicting a whole bunch of people first. Plan ahead where you want them.

Another thing to note is you can't bring in any new dwellers at all if you're at 200. If you get a dweller from a lunchbox or quest, but you have 200 population, there's nothing you can do. To remove the dweller from the line you'd have to be able to bring them in so you could then evict them. This can become a problem as the line is only allowed to be 10 dwellers long, and you can't open lunchboxes period if the line is full. I would suggest not sitting right at 200 population, but perhaps a couple beneath, that way you can remove lunchbox and quest reward dwellers from your line without having to evict any dwellers.

5.7

Q: What are the chances of getting rare/legendary children?

A: The short of it for two max SPECIAL parents is as follows:

Common child:    80%
Rare child:      ~13.33%
Legendary child: ~6.67%

So there are two formulas used to determine this, as follows:

max = (male_total_special + female_total_special - 14) / 126
value = random(max / 4, max)

If value is in the interval [0.85, 0.95) the child will be rare, while if it is in the interval [0.95, 1) the child will be legendary. Anything less than 0.85 will result in a common child. So say we have two max SPECIAL parents, meaning:

max = (70 + 70 - 14) / 126 = 1

This means value will be a random number within the interval [0.25, 1). If we look at the possibilities to the hundredth place, 5 are 0.95 or higher, so 5 of 75 possibilities is ~6.67%. Likewise there are 10 possibilities in the interval [0.85, 0.95), so 10 of 75 possibilities is ~13.33%. From these formulae we can also see where the 122+ for rare and 134+ for legendary comes from. The math is less clean at anything but max SPECIAL due to the formula for max, but it does check out.

This (IHaveACrystalBall) is a very old post that covers it and more in full detail, but the mechanics shouldn't have changed since.