And . . .then there were 6!
Where you come in the order of siblings can have a marked influence on your life choices, writes Geraldine Lynagh
By now, that exercise bike you bought with such good intentions at the beginning of January should be settling nicely into its new role as a clothes horse.
Statistics show most of us will have given up on our resolutions by the start of February, so it is likely that if you vowed to get fit in 2013, you are already shunning exercise and blaming your genes for your sizeable belly.
We can indeed blame our weight gain on our parents. Our genes also influence our height, our likelihood of developing certain diseases and possibly even our IQ.
But there is mounting evidence that we also have our siblings to thank – or blame – for the type of person we become.
We are all familiar with the stereotypes: first-borns are natural leaders; the middle child is the peacekeeper; the youngest the most stubborn and so on.
But you may be surprised to learn that scientific research shows our place in the family also may determine how much we earn, how happy our marriages are and even how often we need to see a dentist. Here is how.
More intelligent: First-born children have on average an IQ three points higher than their nearest sibling, according to Norwegian scientists.
They are generally given more responsibility at an earlier age and tend to take on the role of mentor for their younger siblings, giving them an intellectual boost. Twenty-one of the first 23 American astronauts were first-borns.
Earns more: A survey carried out by Vistage, the world's biggest CEO organisation, found that 43pc of its members are first-borns.
They like to take charge and tend to gravitate towards jobs in politics, IT, engineering and science. They are also most likely to earn a six-figure salary.
More allergic: Researchers in Japan have found that first-borns are more likely to develop reactions to certain foods – 4pc of first-borns had some sort of a food allergy compared with 3.5pc of second children.
They often suffer problems associated with seasonal changes, such as hay fever.
Healthier mouths: Middle children are 5pc less likely to develop gum disease.
It is thought they have stronger immune systems as they are busy fighting off bugs brought home by a germ-loving older sibling.
Lucky in love: Middle children are most willing to work hard at having a happy marriage.
In her book, The Secret Power of Middle Children, psychologist Catherine Salmon says middle children have adaptive skills that make successful relationships. Eighty per cent remain faithful to their partners compared with 65pc of first-borns.
Move far from home: Middle children can feel they have grown up in an older sibling's shadow and can be rebellious.
According to New York-based expert Linda Dunlap, they are more likely to move far from home, to establish a clear identity and escape sibling rivalry.
Shorter and lighter: Researchers in the Philippines found the youngest tends to be physically smaller.
They often have to struggle for attention and are likely to play the clown. Some of the world's most famous satirists such as Voltaire, Mark Twain and Jonathan Swift were youngest children.
Play rugby or ice hockey: Prof Frank Sulloway of the University of California has found that the youngest in a family is more likely to take risks and play dangerous sports.
Another study found they are 50pc more likely to visit A&E.
Earn less: The youngest child is more likely to choose a career they enjoy than one that makes them rich. They are creative and more likely to become an artist or entrepreneur. Last-borns are also less likely to be disciplined by their parents.