Each male pregnancy can trigger a maternal immune response to male-specific proteins. These antibodies may cross the placenta in subsequent male pregnancies, altering brain development and increasing the probability of homosexuality. This is known as the fraternal birth order effect.

The effect of a good caregiving environment is not to make siblings more similar, but to increase their variability by allowing diverse traits to flourish. This challenges the foundation of twin studies, where a lack of correlation between siblings is often interpreted as a lack of environmental influence.

When examining chronic health conditions, older childhood cancer survivors show a striking pattern of accelerated aging. They present with the same rates of multiple co-existing chronic conditions as their siblings who are two decades older. This quantifies the profound and lasting physiological impact of their early-life cancer treatments, leading to premature frailty.

While parents often try to be fair by dividing time equally among children, this practice disadvantages younger siblings. As older children age and require less attention, they still receive an equal share, while their infant siblings get the same amount during a more critical developmental period, giving the eldest more cumulative attention.

According to the "Darwinian niche partitioning hypothesis," younger siblings are often more rebellious and creative as they must differentiate themselves to gain parental investment. With established roles taken by older siblings, they are forced to explore unconventional niches, fostering out-of-the-box thinking.

Economist Joseph Hotz theorizes that parents subconsciously enforce stricter rules on their firstborn as an efficiency play. By maximizing the oldest child's success, they create a role model whose achievements and behaviors will 'spill over' to younger siblings, maximizing the return on total parental investment.

A study by sociologist Emma Zhang found an older sibling's arbitrary academic advantage (from being old for their grade) boosts the younger sibling's performance. This demonstrates a powerful non-genetic, non-parental mechanism through which family-level advantages compound and perpetuate broader societal inequality.

In joint households in Uttar Pradesh, the wife of a younger brother has a lower social rank, leading to her being underweight from doing more work and eating last. This directly results in higher rates of neonatal mortality and stunting for her children, even when controlling for other factors.

Kin detection isn't one-size-fits-all. Older siblings identify kin by seeing their mother care for a newborn. Younger siblings, lacking this cue, instead rely on the duration of co-residence—how long they lived under the same roof with shared parental investment—to develop their sense of kinship.