This publication also mentions it:
http://www.ncjrs.gov/pdffiles1/ojjdp/190106.pdf
If you see page 4 of that publication, the risk factors for gang involvement are listed. The risk factors for delinquency are similar, as are the risk factors for teen parents and/or drug/alcohol use.
If you look at single parents, you will see that they rate a 2.4, which puts that risk factor in the middle of the pack, on par with parental attitudes favorable to anti-social behavior, for instance, in terms of risk. It is not THE MOST SEVERE risk factor, nor is it the least. It is of medium impact.
And, the issue with most young people and high risk behaviors is that it is an ACCUMULATION of risk factors that is really what matters.
So, it is something to be considered. It is not the worst thing in the world, but it definitely has impact.
Thus, single parents need to be aware that it poses a challenge for their children, and take steps to MITIGATE the impact of this risk factor. That can be done by having a high level of bonding with your children (a protective factor), being an effective parent (another protective factor), giving your children opportunities to have pro-social responsibilities and a voice in their family's activities (another protective factor), giving them rewards for positive accomplishment (another protective factor), supporting their school endeavors, etc.
From my perspective, it is important to realize that it exists and assure that your children don't accumulate ADDITIONAL risk factors that will also impact them negatively, if that makes sense.