Are elder siblings smarter?

Read this interesting article about birth-order affecting intelligence in the NY Times. Rather intriguing, since I am generally inclined to believe that intelligence is mostly genetically determined (of course, a really bad environment can overturn genetic advantages...I suppose that a kid who has born into a poor family without access to even basic learning material in childhood might never develop into as smart a kid as he/she could have)

http://www.nytimes.com/2007/06/22/science/22sibling.html

Would like to read more related material before making a judgment