Although many of us would like to believe that birth order has to do with intelligence and personality it isn't that simple. Since researchers have yet to find a strong correlation between the two we are forced to to come up with a simpler hypothesis.
There are many reasons a first born would strive towards achievement namely because they are they are always the first to do anything so they strive to achieve at that and set an example for younger siblings.
Middle children would seem to be diplomatic not because of genetics but because they are often in the middle in fights between the youngest and the oldest. Therefore they must take a diplomatic approach barbecue they are not as big as the older sibling nor are they the "baby" of the family.
The youngest sibling may come off as a risk taker but there are also several reasons for this, the least being genetics. The younger siblings may seem as risk takers but this can be attributed to being the youngest and therefore seeing the older siblings do everything so they know what they can and can't do. Knowing this, it allows them to jump right to the edge because they know from watching their older siblings where that edge is.
And so there are several much easier hypothesis's as to why certain personalities arise based on birth order. It can be easy to say that birth order causes these differences it is much more of a correlation and therefore not genetic.