I wonder how long that's been the case, and if via evolutionary theory it gave rise to physical and indeed cultural gender norms, around strength and competition and such.

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After all, while it's a bit of a stretch, just to hypothesize: in a world with 8 billion people, having a 51/49 ratio means there's 3.90 billion women to 4.1 billion men.

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In other words, there's 200 million men who by definition will not be in a relationship, assuming 100% of possible people engage in an exclusive relationship. That's 200 million men constantly competing to become one of the group of 3.9 billion men. Depending on status, men are able to join the group, or fall out of it. But the default state is to compete, as there's no partner for the bottom rung, which is statistically not true for women. It creates a bias to compete, also in evolutionary terms, which may have caused men to evolve to become physically stronger (an important trait for competition in the last 300 thousand years), and to become biologically or perhaps just culturally geared towards competition.

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I understand this is a super black & white view of the world that enormously simplifies a much more complex world. But I do wonder if this birth ratio difference is one of the drivers of the physical, biological and cultural differences we see (on average) across genders. Would be cool to see a sci-fi show which explores different birth ratios (e.g. 60/40 or 40/60), and what effect that would have on via evolution on biology, and on culture.