Our Gemara on Amud Aleph discusses the relative merits of Lot’s daughters, who for the most part had good intentions, thinking the world had come to an end and only they were left to repopulate the Earth. Nevertheless, the younger one was more modest and chose a name that hinted to her child’s incestuous origins in a veiled manner, as opposed to the older daughter who called her son Moav, which straightforwardly means “from my father.”

The Gur Aryeh (Devarim 2:9) makes a remarkable comment: The younger sibling tends to be more modest as a rule. Rachel too was noted for her modesty in comparison to Leah (Megillah 13b).

Clearly, the Gur Aryeh wasn’t eliminating free will in a person’s behavior. This is reminiscent of the Gemara in Shabbos (156a) that describes character traits a person will have based on the positions of the stars and planets on the day or hour of his birth. The key point is that though the tendency and the character trait are ingrained, the actions that the person will take depend on their morality, which is up to them. For example, it states that somebody who is born under the zodiac of the red planet (Mars) will be driven to spill blood. He may be a shochet, a mohel, a bloodletter, or a bandit. Maharsha notes that these encompass ranges of behavior from sinful, such as a bandit, to a neutral social function such as bloodletting, which is not particularly a mitzvah, all the way to a mohel who is performing a mitzvah. In other words, he is born with a tendency, but what he does with it depends on his morality. So too, it seems the Gur Aryeh believed that birth order impacts the personality and disposition of the child.

(Of course, nowadays we have such sophisticated science and we don’t believe in silly things like astrology. We are much wiser, and so we blame inexplicable mental distress on “chemical balances.” Or, when we don’t know how to explain the mysteries and complexities of sexual orientation and unconscious expressions of symbolic content and yearning via love objects, we declare “they are born that way.” Aren’t we so much smarter these days?)

What does modern psychology say about birth order? Alfred Adler (1870–1937) was one of the first theorists to suggest that birth order influences personality. According to Adler, firstborns are “dethroned” when a second child comes along, and this loss of perceived privilege and primacy may have a lasting influence on them. Middle children may feel ignored or overlooked, causing them to develop the so-called “middle child syndrome.” Younger and only children may be pampered and spoiled, which was suggested to affect their later personalities. Later research claims to debunk the idea that birth order affects personality, although many people anecdotally experience it to be true. Perhaps, since many modern families are not structured in a traditional manner, such as two parents and a larger number of siblings close in age, the newer research does not speak for more traditional families and cultures.

(Source: https://en.wikipedia.org/wiki/Birth_order)