Oldest children tend to be high achievers because they are held to higher standards, especially by their parents. Oldest children are expected to be perfect & adult-like from childhood. Also, they were dethroned & as a result of this, felt in some way that they were lacking, so they ALWAYS have to prove themselves in one endeavor or another.
Middle children either are high or low achievers. Middle children tend to be in the background. They are never valued for themselves but are considered to be appendages of either an older or a younger sibling. It is common for middle children to be anonymous in their families. Some become high achievers in order to be NOTICED while other are resigned to be low achievers because they feel QUITE INSIGNIFICANT in their families.
Youngest children have it both easy & hard. They are the babies of their families hence many are overindulged, even spoiled. Rules are relaxed when they entered the scene. They usually can get away with things that their older siblings would be castigated for. Since everything was done for them, they do not make an effort & use their youngest child status to coast through life. Many are also low achievers because they feel that their older siblings have done before so why even bother and try. However, there are youngest children who want to break the mold; these are the high achievers because they want to MAKE THEIR MARK.
Only children are also high achievers. They also have the HIGHEST self-confidence of any birth order. They are never dethroned hence they do not have to compete w/siblings, having their parents undivided attention. In addition to that, they grow up in an adult environment so they adopt adult mannerisms & behaviors early. Since their primary interactors are their parents, they develop an adult outlook on life early. They also have the intellectual & cultural opportunities which children w/siblings seldom have. They also have the economic resources such as monies for education esp. tertiary education. Only children have very high, even perfectionist standards which makes high achievement possible.