"Such effects are minuscule and can be explained by the fact that first-borns come from families that are smaller, and more likely to be higher in social class, than the families in which later-borns are raised," said Frank Sulloway with the Institute of Personality and Social Research at the University of California, Berkeley.
Hilfer also says the points in Blair's book are probably on target but stresses that birth order shouldn't be a factor in finding the love of one's life.
"I don't think people should use birth order to choose a person to fall in love with."
Others find that the book relies too much on generalizations that can mislead.
"For example, she says that last-born are often are somewhat dependent, and that is true, but there's a lot of misguidance in her directing about birth order matching other birth orders and making them compatible couples," says Fran Walfish, a Beverly Hills, Calif., family and relationship psychotherapist who is also author of "The Self-Aware Parent."
The University of Texas at Austin's Falbo said birth order doesn't have as much of an impact as others believe it does.
"We like to think of birth order as being really important. It's like astrology -- we can read the forecast and think maybe we get something out of it," she says.
"By no means does [birth order] explain so much else that comes from our families of origin, 'our baggage,' that we take with us," said Sharon O'Neill, a psychotherapist from Westchester County, N.Y. ,and author of "A Short Guide to a Happy Marriage."
Rather than focus on factors like birth order, experts suggest relying on something much more important.
"Nothing matters when you fall in love. Throw all caution to the wind when that happens," said Hilfer.