Can 2nd cousins have children?

First cousins can have children. Siblings can have children. Uncle-niece; father-daughter; etc. 

As long as there are no genetic diseases in the family, degree of relation is actually not an issue. It is more of a social issue. Biologically-speaking, the presence of genetic diseases is the determiner. Socially-speaking, each culture has different taboo relationships.

The reason: If there is a genetically transferable disease, and we do not know whether it passes through an x or y chromosome, we assume that each child of a single-carrying parent has a 50% chance of transmitting it to their offspring. So, when you take two siblings who may be carrying it, you theoretically increase the chances of their offspring having the disease. Testing for the genes lets you tell with greater certainty.

First cousins introduces 2 fresh sets of genes into the equation, but if both of the siblings (uncles/aunts) are carriers, you have the same situation, basically, as siblings with one carrying parent. Now carry that down to second cousins. It would depend on whether your mother or father was a carrier, and whose first cousin (your mother's or father's) you are considering mating with.

The advice in rabbit breeding for real-life purposes is you can breed siblings once every 3-4 generations, but otherwise, you need fresh genes.

Thanks for a very thorough reply. I was thinking of sims in my TS4 game, though, and I guess the programming gets the last word. I had a couple marry, but they could not have children, so there are combinations where two siims can get married, but there will be no option like "try for baby". In such cases, it would be nice (and time consuming for both me and the sims involved) to know these things in advance.

There must be some rule in the game, no? I wish we could look it up easily. Or, is the mechanics a bit wonky - like relatives can not have kids no matter how distant they are?

I'd like to know where this game sets the cut.

Sounds like something EgonVM might know.

I didn't know they had that in the game. Maybe you could use the science baby option.

I have been summoned!

Unfortunately, through tuning files, I was unable to find any answers, so I had to resort to in-game testing.

So, after looking up the definition of "second cousin" (English isn't my first language), I created a test family and tried to replicate the definition. So I made a sim named "a", her father, her grandfather, her grandfather's sister, her grandfather's sister's daughter, and her grandfather's sister's daughter's son aka the second cousin (jeez that took long to write out).

The results:

I could set a's relationship with her second cousin as spouse in CAS. This however removed a's family ties, so I went and tried to get them married through gameplay.

They could romance each other. They could get married to each other (thank the Lovebug's early proposal for bypassing the 6 hours after first kiss rule!)

They could try for baby.

So yeah: final answer: yes, sims can have babies with their second cousin. Grandparent's (and greater's) siblings won't be considered as part of the family anymore, thus the game allows it.

Proof: The family tree:

TLDR: Yes

I was asking for my Crown Princess, in thise circles family tree is important, and I don't think her parents the King and Queen would applaud a science baby. Thewy have a couple candidates for her, but one is her second cousin, and if they could not have kids there would probably be no approval to the marriage either.

Thanks a bunch for testing! I'm a tad confused though as I did see several occasions in my game where a couple can marry but can not try for baby. Maybe there were other reasons than distant relationship though, I hardly recall who they were...

Or, if this had been made easier recently, through a patch, maybe those who could not try for baby back then, would have been able to try now. Guess I'll never find out, LOL.