I recently read the book Divergent* as a preview for my son. That post-appolocyptic story is all about a culture where people are 'stuck' in one of 5 lifestyles (or factions) according to their basic temperaments. This is not a new concept. Fascination with temperament date back to Ancient Greece, and remained a dominant way of describing and explaining personality before modern psychology emerged in the 1900s.
Although the notion that behaviors are caused by excess body fluids was not correct, their observations of the characteristics of these four temperaments has remained and for centuries was a primary way of discussing personality.Modern developmental psychology does not have much reverence for temperament theories in general and consider them to be a psyudo-science, yet they do have a place in history and have influenced how we think about the person, choosing to emphasizes the 'traits' of a person rather than a 'type' of person.
Outside of developmental psychology, some areas of education (for instance Waldorf approach) find value in seeking to understand a child's temperament. The Church too, has found that understanding the temperaments can be helpful in guiding the individual to grow in holiness. When one is able to recognize that s/he has a tendency toward sins of a certain nature, then the individual is better able to combat those sins.
Dr. Sulloway is actually a historian of science. I loved his book, Born to Rebel (Sulloway, 1996) in which he examined the theory that researchers who challenged established views tended to be later borns, while those who support the status quo tended to be firstborns. I mention Sulloway, because although he is not a psychologist, he provided some solid evidence that sociologists had been seeking.
It is impossible to break apart the genetic part from the environment part. As David Moore points out in his book The Dependent Gene: The fallacy of nature vs nurture, every gene must be expressed in an environment, and it is impossible to tease apart the influence of the gene from the influence of the environment. We see this in pregnancy so clearly - we are soooo careful about everything we do or eat, because we want to make sure that environment is optimal for that developing child. And little blips in the environment, not enough folic acid, or exposure to various toxins, can influence our little ones on the cellular level.
One specific way birth order has helped my husband and I to understand and appreciate our kids better, is to think about those qualities we each have that our children also have. I can see some first born qualities that I love in my husband and look for those those same qualities in my eldest son. My husband can identify some characteristics in our middle children that I also have (since I am a middle child), and we can just laugh about those. Sometimes there are things that make us crazy in our kids, but they are qualities that we value in adults around us - stubborn becomes tenacious when it is a trait we find in our spouse!