Many western democracies limit the power of their politicians to control the electoral boundaries to avoid the possibility of gerrymandering. UK electoral boundaries, for example, are redrawn when required by an independent commission. Many US states, however, put the boundaries entirely under the control of elected politicians and this often results in some fairly odd boundaries.
Some regard this as self-evidently bad. For example, the following argument is made on www.fairvote.org:
Redistricting encourages manipulation of our elections by allowing incumbent politicians to help partisan allies, hurt political enemies and choose their voters before the voters choose them. The current process is used as a means to further political goals by drawing boundaries to protect incumbents and reduce competition, rather than to ensure equal voting power and fair representation.
Some states have had non-political control for some time (Iowa, for example) and others have recently enacted citizen driven initiatives to remove the power from politicians (eg California). (see this CNN report for some examples).
It seems likely that gerrymandering will strengthen incumbents. But what does the actual evidence say? How much does it increase electoral security for incumbents?
Note: The original question here asked about both strengthening incumbents and driving polarisation. I've now separated this into two questions so the two issues can be addressed separately. Hopefully this resolves some of the confusion expressed in the comments on the original.