http://www.telegraph.co.uk/science/science-news/11171567/Season-of-birth-affects-your-mood-later-in-life.html
In my family, four of us have birthdays in January. I am the less irritable and less emotional, but the other three-oh my goodness.
The one born in April is positive, but the one born in May is, well, phlegmatic.
And, the one born in September is one of the three moodiest of all.
There have got to be other factors...