This study I recently found sort of actually contradicts an old idea that have been going on.

Study #1: The association between height and birth order: evidence from 652 518 Swedish men.

Results:

Unadjusted analyses showed no differences between the first two birth orders but in the fixed effects regression, birth orders 2, 3 and 4 were associated with 0.4, 0.7 and 0.8 cm (p<0.001 for each) shorter height than birth order 1, respectively. The associations were similar in large and small and high-SEP and low-SEP families, but were attenuated in recent cohorts. Birth characteristics did not explain these associations.

CONCLUSIONS:

Birth order is an important determinant of height. The height difference between birth orders 3 and 1 is larger than the population-level height increase achieved over 10 years. The attenuation of the effect over cohorts may reflect improvements in living standards. Decreases in family size may explain some of the secular-height increases in countries with decreasing fertility.

Analysis

Now this is just one study but the study seems to be done on over half a million Swedish men where the measurement of height was taken when they were just 18 years old, which is believed to be the average age when the vertical growth in males should have stopped.

The old idea that was stated by other scientific studies was that the siblings and kids who are born later like child #3 or child #4 would end up to be taller on average. The rational for that was that the uterus for the mother during the first pregnancy (and maybe also the 2nd pregnancy) would have been smaller than the subsequent pregnancies. The size of the fetus or baby before birth is said to have been determined by the size of the the uterus or womb it was carried in. Maybe to the average person without anytype of obstetric training, it might be ‘common sense’ to think that once the baby before birth pushed the womb up to a certain size, the uterus will start to go through the contraction signifying that birth is supposed to occur.

There are already some studies that show that larger females in terms of taller mothers and mothers with wider hips do give birth to heavier and taller babies. In some old posts I had used the logic that the size of how big a person will become may be mainly determined by how large they are at the moment that they are born. My logic back then which seems to be proven false from this study is the idea that the larger the uterus and womb was from the start the larger the baby will be before it actually comes out.

I would have guessed that children born later in order should be bigger. However this study says that the earlier children, the 1st and 2nd child of a couple seem to become taller. Why?

What is noted is that there might be a social-economic correlation between decreased family size and increased children sizes. This might mean that the size of a child can be partly determined by the amounf of resources (ie food, time, energy) the parents of the child can give to it on an individual basis. Perhaps as the first and second born, the child gets more attention and energy from the parents than if they were the last of a ‘litter’ and the decrease in resources due to resource sharing alone from having so many siblings is all that is required to end up just slightly shorter than one’s older siblings.