A lot of people in this thread don't understand complexity as a function. First off, this is a working theory, not a scientific law. So it needs to be tested to see if it is verifiably true. Second, NO ONE IS SAYING THAT BIRTH ORDER IS THE ONLY FACTOR INVOLVED. The study is saying that it increases the probability of being gay. There are almost certainly a large amount of factors that go into determining a person's sexual preference. Societal acceptance, genetic hormonal/mental baselines, hormonal responses to external stimuli, shaping events during adolescence and young adulthood, and perhaps even things like diet could be other contributing factors. However, those factors would have to be studied individually in order to confirm or deny suspicions, and they would still be measures of probability, not certainty.