Mangal Dosha (also Kuja dosha or Manglik dosha) is said to arise when Mars is placed in certain houses of a birth chart. Mars is the planet of energy, heat, argument and drive, and the belief is that in the houses that touch partnership and domestic life it can bring friction.
Which houses count?
Mars in the 1st, 4th, 7th, 8th or 12th house from the lagna makes a person Manglik in the most common North Indian rule. Some South Indian traditions also count the 2nd house. Many astrologers also check the same houses counted from the Moon, and a few from Venus.
Cancellations
The tradition lists many situations in which the dosha is cancelled or reduced. The most accepted are: Mars in its own sign (Aries or Scorpio) or exalted sign (Capricorn); Mars in the 1st house in Aries, the 4th in Scorpio, the 7th in Capricorn, the 8th in Cancer or the 12th in Sagittarius; and Mars joined or aspected by Jupiter. If both partners are Manglik, the two are considered to balance each other.
How common is it?
Very. Because five of the twelve houses count, roughly 40 percent of charts have Mars in one of them by the strict rule, and more if you also count the Moon-based check. That is part of why it is so often talked about.
How to approach it sensibly
Treat Mangal Dosha as one factor among many. If one partner is Manglik, look first for cancellations, then at the rest of both charts. Don't make decisions out of fear and don't pay for expensive rituals. Honest conversation about temperament, family and expectations matters more than any dosha.