The bigger the better? This statement might not be true when it comes to family size. According to the Lilienfeld textbook, the claim that older siblings may have higher IQ has yet to be completely proven, but there may be some semi valid proof involved already.
When a couple has their first child, this child does not have to "compete" with other siblings for any attention. Therefore, they have the advantage to grow both physically and cognitively and expand their knowledge without restriction. However, when another baby shows up in the picture, not only does the first child have to start competing for attention and opportunity for growth in their respective stage of growth, but the new baby must receive the blunt of the first child receiving attention when the baby needs an equal amount of attention, and therefore is set back developmentally, even if ever so much. The more children a family has, the more likely the younger children to not receive enough attention and suffer the effects via mental development (IQ, social skills, language onset).
The textbook states that couples with a lower cumulative IQ do tend to have more children and so, this may be an ongoing cycle where low IQ parents have lower IQ children (both from environment and genetics), and those children, being raised in typically large families, find it socially acceptable to have large families, and continue the cycle. Although the correlation between IQ during the early years of life and adulthood is quite low, the resulting competition of older siblings in later stages of life may stunt those children in more important stages of life and so, this debate has yet to come to completion. Older siblings may very possibly be the ones getting the short end of the stick.
Another topic that involves birth order is the claim that where you are in the birth order lineup contributes to your personality traits. The textbook only touches on this concept and then dismisses it due to some lacks of reliability in experiments. None the less, the concept of birth order determined personality is intriguing and may still be correct or incorrect, so for now, I will still discuss some proposed differences in personality. How do you measure up to the "findings"?
First-born children are said to be "natural leaders", and are picky, precise people. They like seeing things done right. Some first-borns take an alternative route and are known as the "nurturers". They like to take of their siblings. On the downside, they are also known as "bossy", "intimidating", and "know it alls".
The middle-borns are typically claimed to be the "rebels" of the family. They may try to be the exact opposite of their older sibling. They lean toward their peer groups more than family because they feel they " do not belong" in the family due to the claim that their older sibling gets all the glory and the younger sibling gets away with whatever they want, yet they seem just...average. They tend to be secretive as well and not share what they are feeling with just anyone. Being a middle child myself I can actually relate pretty closely with these claims. My two brothers do seem to always be attracting more attention than me and sometimes in my childhood I did feel "lost and forgotten".
The youngest of the family (whether it be the second born or the twenty second) are usually the most social and outgoing. They are less likely to have financial security due to "frivolous" tendencies. They are usually "spoiled" and can twist their parents by the ear and get whatever they want. As mentioned before, even if a second child is technically the middle and last child, they may gain some tendencies from both groups, just as an only child may develop attributes from all three groups.
Although these profiles have not been completely proven to be true, the media and celebrities in the spotlight have backed up these claims by portraying personalities similar to the stereotypes. Only time and advances in technology will be able to tell, if ever, if these statements are actually true, both IQ and personality claims.
Sources: Lilienfeld textbook chapters 9 and 14