LIVIA started following Westerossi are tolerant of incest in Targs, but no one else, Why women don't take black, Daemon Blackfyre won the Battle of Redgrass Field and and 3 others
LIVIA replied to Quorra's topic in A Clash of KingsGRRM told at least twice that Targaryens indeed suffered from the consequences of incest (if links are needed I can provide them). But really serious troubles apparently started after the death of dragons. Apart from the fact that dragons seems to have positive impact on their riders' health, their extinction made the Targaryens more dependent on Vesterosy nobility and urged them to marry out of their own house for political purposes. And that was it. If both you and your brother have perfect genes, you can couple without any serious risk for your children, and so do they, and so do only-your grandchildren. But if one of your descendants decides to marry a girl from another family, you cannot say if his bride has perfect genes too. If not, their children will be already recommended to avoid inbreeding. We know about at least five such causes in Targaryen family: 1) Aegon, Visenya and Rhaenys, children of Valaena Velaryon: very weak and sickly Aenys and sadistic Maegor. 2) Viserys and Aemma Arryn (his first cousin with Arryn blood): multiple miscarriages, one cradle death, a single survived child. 3) Aegon II and Helaena, children of Alicent Hightower: weak and mentally challenged Jaehaera and six-fingered Jaehaerys. 4) Aegon IV (Aemon) and Naerys, children of Larra Rogare: only two of six children survived, the son lacked height and strengh. 5) Jaehaerys and Shaera: two children, one of them mad and probably having problems with fertility. So, the remaining Targaryens have either to carry perfect genes of Valyrian dragonlords or to give up their old tradition. And to be honest, I don't favour the last option, because too many dragonriders from all the Vesteros are fucking much more dangerous than too few.