The linked video is an example of a father (John) having a romantic relationship with his own daughter (Jenny) and having children with her. The documentary also introduces viewers to a romantically involved half-brother and half-sister. But doesn't nature rig close relatives so that they are sexually repulsed from each other? Yes, but only if they live in close proximity during a critical early developmental window. This potential desensitization to sexual attraction is referred to as the Westermarck Effect. In the case of John and Jenny, the daughter had essentially no contact with her father for the first three decades of her life. Same situation with the half-siblings. Without the Westermarck effect to pull back on the reins, "genetic sexual attraction" kicks in to supercharge the romance. Notice how the moralistic and legalistic discussion in this documentary runs orthogonally to the biological research. Not once is the Westermarck Effect discussed, even though it sheds substantial light on these situations. It often occurs to me that we'd be better off analyzing social situations in terms of evolution and ecology in addition to legality and morality, but that would deprive us of so many opportunities to engage in angry finger-pointing and judgmental barking. To consider the science would admittedly require some effort, something that many of today's self-assured people are unwilling to do. If people did take the time to think things through more rigorously, however, they would likely see that this "father" and this "daughter" are dramatically unlike prototypical fathers and daughters in dramatic ways that correlate to solid biological and psychological research. If they took the time to understand this situation using (easily available) science rather than simply folk-morality, even the harshest critics of these couples might have the following thought: If I had been in that situation, these same sorts of powerful attractions might have overwhelmed me too. A perspective infused with even a bit of science would have set a different tone for this entire documentary. A bit of scientifically-informed self-critical thinking might even open the door for a more empathetic perspective. It's a new multidisciplinary world out there with regard to "morality," as psychologist Jonathan Haidt eloquently explains at Edge.