Dan,
like the photo and the "?". A picture speaks a thousand words.
Explanation:
You can see clearly the slight anhedral (which is laterally destabilising). However, remember that aircraft also has:
- T-tail
- Rear side-mounted engines
- Swept wing
all of which are laterally stabilising design features. If you added dihedral as well you would either have an aircraft with a roll-rate that would make a Cessna 152 look like a world-class aerobatic aircraft, or you would need massive ailerons and probably roll spoilers as well, neither of which are particularly desirable design outcomes in a commercial jet...
