Do the Eldest Boys Have the Advantage?

Norweigian researchers debunk the assumption that the eldest boy in families has the IQ advantage by presenting a more complex story. Boys who grow up as the "senior" child in the family have a higher IQ on average than other siblings. It's a fascinating finding because it really brings back the debate of nature vs. nurture and it reminds us that nothing is ever black and white, but an indiscernible series of grays.

They studied the IQ results of 240,000 men drafted into the armed forces over a 10 year period and found the following:
1st born: 103.2
2nd born:101.2
2nd born if 1st born dies: 102.9
3rd born: 100.0
3rd born if 1st & 2nd die: 102.6

A few concerns I have about the conclusions:
-- Are the differences in the IQ scores statistically significant? Perhaps they are, but the differences seem very slight to me.
-- How do girls rate by comparison? I can understand that given their source of data (drafted men aged 18-19), a comparison simply wasn't possible. However, it would be interesting to see the differences. Do girls fare as well in a similar situation? Does the eldest boy fare differently if he's the elder of sisters only? The article notes that some such studies have been conducted with conflicting results. Is it possible that the boys' higher IQ scores are, as the article suggests, caused by his role in the family? That is, he has to help younger siblings with various tasks including homework. If data on girls is conflicting, perhaps it's because not all families expect the eldest girl to help younger siblings in an intellectual.

One more thought. The article mentions that "the older child benefits by having to organize and express its thoughts to tutor youngsters." This is interesting to me because I've heard of a classroom learning technique where two different grades are mixed together so that the younger student can learn from the older, and the older is challenged to reinterpret something learned so as to explain it to someone else. Personally, I believe that's an incredibly effective method of learning because I know that if I can repeat something effectively in my own words, I've understood the concept, not just the words.