There are a several numbers, animals and objects that are considered unlucky. People avoid these “unlucky” things that they think will bring bad luck. In Japan, people believe that the numbers 4 and 9 need to be avoided due to some reasons, such as the number 4’s pronunciation being similar to the word death’s pronunciation in Japanese. Number 13, which also means death, is considered unlucky by most people too. In some places, it is believed that black cats bring bad luck, which is nothing but a superstition.
When we do a research about these things bringing bad luck, we won’t be able to find a reasonable explanation for any of it. It doesn’t make sense at all that a number, a simple object or an animal, a living organism, can bring bad luck. Therefore this kind of thoughts and theories are not true, especially when there’s no proof.