How did Jon Arryn figure out that Joffrey, Myrcella and Tommen were incest bastards?

[sammy~two~swords]

must be true , stannis isn't the sort to lie . I wonder what made him go down that road....look at all the bad shit it caused . Sometimes a lie is better than the truth , I think that's the case here

[Attitude]

Also a possibility, but as you say that leaves no room for discussing the character's motivation.

Uhm, what?

I mean that Cersei admitting the truth to Ned, leaves no room for dicussion on whether or not the kids are Cersei+Jamies. Cersei was not lying there. Joffrey, Myrcella & Tommen are kids born from incest between the twins Cersei Lannister and Jamie Lannister.

[Mikkel]

Uhm, what?

I mean that Cersei admitting the truth to Ned, leaves no room for dicussion on whether or not the kids are Cersei+Jamies. Cersei was not lying there. Joffrey, Myrcella & Tommen are kids born from incest between the twins Cersei Lannister and Jamie Lannister.

Oh. Thought you meant that it was GRRMs decision to unambigiously confirm it to the readers. Carry on then, nothing to see here.

But yeah, I agree there's no room for discussion. Won't stop certain people from denying it though.

[Hoosier427]

Jon Arryn actually had 3 of Robert's bastards to look at. 2 boys and a girl, just like Robert's natural children. We have Edric Storm, Gendry, and Mya Stone who lives in the Vale. Compare those three to Robert's natural children with Cersei and it becomes easy to see that they don't belong to Robert. the only feature Cersei children have of Robert Baratheon that would even allow you to guess that the kids are Robert's is that they have his last name. That it. Nothing else says Baratheon in those children.

[Lykaios Wolfsheart]

This doesn't actually make a lot of sense. Let's say Stannis got suspicious that Cersei had been cheating on Robert and the kids were not his, due to Robert's comments and the fact that all of Robert's bastards looked like him (not that this is any proof - his kids with Cersei could still happen to look just like their mother).

But for all three to look identical to their mother? When all of Roberts other children carried his features to the core? I mean, Gendry's mother had blonde hair (I think the quote is "She had yellow hair" when he's describing her), and he's the spitting image of Robert. I think with 16 bastards who all carry his features quite heavily, his three "true born" children having the Lannister look is way more than suspicious.

And to be honest we don't actually know that he was investigating incestuous relations between the twins, just that he was investigating evidence of the children's illegitimacy. Someone clearly tipped him off about the incest bit.

[WightsNatch]

I think it all comes down to logic and reasoning. Jon Arryn sees and takes care of black-haired, blue eyed Baratheon bastards while at court he only sees golden-haired, green eyed Baratheon trueborns that look like Lannisters. Stannis has Edric Storm, who looks like Robert. They visit and see Gendry. He looks like Robert. That raises suspicions.

What is the next step? If they think the Queen is unfaithful and knowing Cersei's personality, who'd they suspect for the parentage of the royal children? Who is around for a lot of years (three separate pregnancies do take quite some time - and usually a woman doesn't get knocked up three times from having sex on just three occasions) and has virtually unlimited access to the Queen? Barristan Selmy? Varys? Are you kidding me? Jon Arryn and Stannis know all about Targaryen incest, so it isn't too far-fetched to include Jamie Lannister in their list of suspects. After all, the kids already look like Lannisters.