Over time, the wing aspect ratios of commercial airliners have increased.

For evidence ,see the following data:

* 1980s:
  * Boeing 747-400: 7.91, 
  * Boeing 757-200: 8.0 
  * Boeing 767-300: 8.0
  * Airbus A310: 8.8 

* 1990s:
  * Airbus A330: 10
  * Airbus A340: 9.2
  * Boeing 777: 9.96

* 2000s: 
  * Boeing 787: 11

The advantage of a higher aspect ratio wing is that it increases L/D ratios by reducing induced drag, and thus for a given amount of lift, an airliner will incur less drag and burn less fuel. However, this is traded-off against the fact that the wing has to be thicker to counter the increased bending moments from the longer wings. This eats into the drag-reduction fuel savings. 

But over time, it appears that engineers have managed to overcome the trade-off, making the wingspan longer *without* incurring the penalties of increased weight. How have they managed to do this? 

Two notes: 

1. On the most recent airliners (eg, the 787), there's considerable evidence that the shift to carbon composite wing structures has allowed for this to happen. However, the shift began before composite wing structures were introduced, so there must be other explanations 
2. I'm almost sure Finite Element Modeling plays a role, but I'm just not sure *how* FEM allowed for the increase in AR without incurring the offsetting drag-increasing penalties.