A new study shows that about 1.7% of fathers who have no reason to doubt that their children are biologically related to them, do not in fact have a biological tie to their child. Previously, a 10% figure was often quoted. But, apparently, the study that produced the 10% figure included many parents who had good reason to know that they might not be biologically related to their children.
It is also worth noting that in Colorado and many other states, paternity is more than biology. Biological parents can have their rights terminated by operation of law through the passage of time (sometimes paternity cannot be legally adjudicated after a child becomes an adult), by a presumption of paternity that becomes conclusive in someone else, or by courts for neglect or absence. Adoption can establish a paternity relationship between a parent and a child in the absence of a biological tie. And, paternity is presumed if the father is married to the mother.
Controversially, but correctly IMHO, the marital presumption becomes conclusive after a certain number of years (often five), even if it can be definitively established that there is no biological relationship between the parent and the child. Also, standing to seek custody, which is the most important right flowing from paternity, can arise simply from an extended period of time in a parental relationship with a child.
Also controversially, and often incorrectly IMHO, standing to seek custody can often arise simply from having "possession" of a child, a provision which is not nearly narrowly tailored enough to do justice to all the circumstances it can encompass.