Good question. I can only offer my opinion, no research.
In my opinion, it seems as though doing this is mixing 2 separate actions on one element (i've done it myself in the past).
I've come to the conclusion that the navigation click action on the "Services" item should be removed. You will face further problems when people use touch screens. E.g. when they tap the "Services" item will you open the sub menu or navigate to the Services page? This could be a big problem if those sub pages are only accessible from that drop down.
Some examples of solutions to the problem:
- Have an option on the sub menu called "All Services".
- Split the button in 2: a larger part that is the link to the Services page, and a smaller part with an obvious arrow and border that opens the submenu (from Nate Kerkhofs in the comments below)