birth order is quite fun. it is both completely without value, and highly important. How is that?
Well, in a mixed sample, my ability to predict your IQ, personality or any other reliable individual difference based on birth order is quazi-zero.
However, if we compare population effects (say the difference between
1920s and 1990s) the proportion of first borns etc explains the bulk of
the variance. The theory is a bit complex. I am first born, then the
mean IQ of my environment is quite high of the first few years (mom, dad,
and me). Then along comes my brother. His mean situational IQ is lower
(it includes me ;-). Mine has also decreased, however I now have a
language acquisition device trying to acquire knowledge from me (my young sib). This is great for me to learn: he pesters, I find out. I have to really find out otherwise he pesters me even more. So, being first born is a plus. The theory goes on to deal with cross over effects
(advantageous for years 1-4, then relatively disadvantageous: for
instance my young brother might get a leg up when he is starting school by bouncing his new experiences off me).
Anyhow, the real fun starts when you map these demographics out to school and society: you can get virtuous circles in which classes of young learners boot-strap each other - gen-X goes on the Internet for instance. While these situations can also lead to dark ages and depressions, they are precedents (from our view) for the singularity.
So, the theory is not about birth order at all. It is about situational learning and birth order affects our situation by altering the surrounding information sources and sinks.
On a more speculative note (did he say MORE speculative ;-) my feeling is that the next 20 years give us a chance to hit the singularity, but that if we miss, life will not be fun. The world will be enter a vicious circle of old age, lack of output, lack of investment and general parlous disaster 1998 Japan * about 25 ;-(