Baransu Byou, Chunin exams

First law of chakra:

The chakra is formed by two part, the mental one, known as Yin Chakra, and the body part, or Yang Chakra. Women and men are both able to use chakra, with a small problem, known as Balance disease or Baranzu Byou.

Men suffer from abundance of the Yang chakra, which makes them more impulsive the more abundance there is, causing them temperament problems.

Women suffer from a shortage of the Yang chakra, compared to their Yin chakra. This makes them more prone to discomfort and mood swings.

According to the religion of the creator of ninjutsu, whose name is no longer remembered, but  whose teachings are still known, such as a law inscribed in the same being of the users of energy was established by them.

The law of balance, or law of the chakra.

This is an impulse or suggestion of the subconscious of the shinobi to match the amounts of Yin and Yang chakra in the body. For this, the method  is quite simple.

The woman and the man must copulate.

Throughout history this caused great problems when in the midst of battles between enemies, the Shinobis suffered Baranzu Byou. The orgies produced were not something that people takes pride on, but it was necessary.

Blood relations, age or race differences were irrelevant to the problem.

Over time, the foundation of the Ninja Villages solved the problem to some extent, as now they didn’t have to resort to incest or copulating with enemies to survive.

The creation of the teams was the best solution so far.

Mixed teams allowed the option of balancing the members' chakras without problem, in order to be in the best possible state at any time.

The problem began at puberty, fortunately, and the elderly lose the need to deal with Baranzu Byou as they get older, so the people with most trouble are the teenagers and young adults.

Thus, the ninja world developed normally.

With small differences, of course.