I had horribly crooked teeth and my mother has them too. I was told that I used to touch my gums with my fingers that's why my teeth were crooked.
What are the facts and myths regarding crooked teeth and what are the preventive measures?
The truth seems to be that we don't fully understand all the reasons why one's teeth may grow in crooked.
There are a number of theories, including one that hypothesizes that the change from our early ancestors' hunter-gatherer lifestyle to the modern agrarian lifestyle has resulted in a gradual shortening of the jaw, resulting in less room for the teeth to grow (which, in turn, results in crooked teeth as teeth compete with each other for space).
As far as facts, the only authoritative references I found were in a release by the American Dental Association and the American Academy of Pediatrics Dental's 2013 Fast Facts document (please forgive the American resource bias; they just happen to be the first names I think of when looking for reputable scientific resources).
They both agree that the biggest factor is lack of early childhood dental care.
Both mention that Early Childhood Caries (a.k.a. Baby Bottle Tooth Decay), defined as dental caries (cavities) that occurs in the primary (baby) teeth of young children, is a major contributor to crooked teeth.
If a primary tooth is lost due to tooth decay, the teeth adjacent to the lost tooth may drift into the gap in the jaw, causing the adult teeth to subsequently grow in crooked (or even prevent them from erupting). Healthy primary teeth, on the other hand, help guide the newly erupted permanent teeth into a straight position within the jaw line.
The AAPD's source also mentions two other factors.
The first is poor sucking habits from a pacifier or thumb:
Additionally, early intervention by a pediatric dentist can assist in the development of permanent teeth, and prevent them from growing in crooked:
So, in short, the best preventative measure is early and regular visits to a pediatric dentist.