No, says Dr Patrick Ryan, Director of Clinical Psychology at the University of Limerick, who believes that the number of children in a family doesn't have a major impact on how a child turns out.
Instead, he says, it's the kind of parenting you receive which has the most influence.
"On average, the number of children in a family does not seem to have as big an effect as we commonly assume.
"The key variable in how children turn out is how their parents interact with them. It's the quality and sensitivity of the parenting they receive which matters," he says.
Ryan explains that research shows that only children tend, on average, to be more highly motivated and tend to score better on IQ tests -- possibly because they can get a lot of intensive support from parents.
However, he adds, studies show that in terms of happiness there is actually very little difference between an 'only' child and a child from a large family, even when you look at them as adults.
The Understanding Society report, which claims that happiness declines the more siblings there are in a household, says about 30% of the teenagers surveyed complain of regularly being called "nasty names" by brothers or sisters while 17.6% say they have their belongings taken away from them.
But that's not necessarily a bad thing -- strife amongst siblings can teach children some important life skills, says the support group Parentline, which often receives worried inquiries about sibling squabbles.
"We get calls quite regularly from parents who are stressed about constant sibling rivalry -- in fact, the problem is so common that sibling rivalry has a category of its own in our records," says Rita O'Reilly, who believes, however, that the aggro can have an upside.
"Most of the carry-on with multiple siblings, the taking of toys, the squabbling and the jealousies is actually good for children because it prepares them for the world outside and teaches them to get on with people and engage in arguments, etc, and not to take it personally."
Only children don't have to compete for parental attention, they don't have to share their toys, they have more privacy and are not subjected to sibling fights -- but on the other hand having brothers and sisters has benefits.
Siblings can teach us crucial lessons about life and relationships, says clinical psychologist Dr Deirdre MacIntyre, director of the Maynooth-based Institute of Child Education and Psychology Europe.
"Having siblings gives us the opportunity to learn very important life and relationships skills -- eg, caring for younger children. Smaller families mean that a lot of the time children today don't have the experience of changing nappies or making bottles."
MacIntyre discussed the issue with her three children aged 10, 15 and 17 -- and while they agreed that more children in a household generally led to more squabbling, they believed that on the plus side, there was "company and fun" and said they couldn't imagine life without their siblings.
Later in life, adds MacIntyre, good sibling relationships can provide support in many ways, such as with the care of ageing parents.
"You'll often see that brothers and sisters who were always fighting as children become great friends in their late teens or early twenties," she comments.