If you’re the firstborn of the family, you’re more likely to suffer from food allergies and hayfever than your kid brothers and sisters. One possible reason, experts say, is the hygiene hypothesis – the idea that less exposure to bacteria leads to weakened immune systems.

“Firstborn children spend their infancy in a much more sterile environment than those born later who are exposed to the rough-and-tumble of their siblings’ play,” explains Professor Michael Alpers, from the Centre for International Health at Curtin University.

“This is why there is more allergy in the children of the First World than in those growing up in Third World poverty.”

The moral? A little dirt doesn’t hurt – in fact, it may be good for you.

If your sibling suffers from cardiovascular disease, there’s no question that you are also at risk. If your brother or sister has a stroke, you are up to 64 per cent more likely to have one too, says Professor Stephen Harrap, assistant dean of biomedical sciences at the University of Melbourne.

If your sibling has a heart attack, your risk skyrockets to up to 500 per cent.

Regardless of whether they’re older or younger, if your sibling has a stroke or heart attack under the age of 55, “that is a real signal that there could be something special in the family that predisposes you to such premature cardiovascular disease issues,” says Professor Harrap.

“It’s a warning that you should check your own risk status by seeing your doctor, getting your blood pressure, cholesterol, weight and glucose levels measured and make sure you’re in as fit a shape as possible.”

All family history plays an important role in cancer risk, but experts believe that when siblings develop the same cancer, the lifestyle they shared when growing up is an important factor.

“Certainly if you get sunburnt as children, then both of you have the same risk factor for developing skin cancer later on, because of the potential damage done,” suggests Cancer Council Australia chief executive Professor Ian Olver.

“Or you might have similar attitudes to alcohol, you both might smoke if you grew up in a household of smokers, you may have had similar diets and rates of obesity, so that may make you more prone to the cancers related to that.”

Breast, ovarian and bowel cancer are another matter. “There are genetic mutations for breast cancer [such as BRCA1 and BRCA2] that run in families,” says Professor Olver. “If your sister has one or two mutations for breast cancer, you need to to be tested because if you also have it, you have an 85 per cent risk of getting breast cancer, and a lower risk of ovarian cancer.”

Similarly, your risk of bowel cancer doubles if your sibling aged 55 or over is diagnosed, and increases up to six times if they’re under 55. As with any cancer, early detection is crucial to long-term survival, so if your sibling is diagnosed, don’t delay getting tested.

The number of siblings you have the order in which you were born have a fair bit to do with your chances of developing inflammatory bowel disease, such as Crohn’s disease and ulcerative colitis.

As with allergies, the hygiene hypothesis is thought to play a part – lots of kids in the family means more potential exposure to the pathogens that help build your immunity to bowel infections later in life.

Genetic factors also count: if your brother or sister has Crohn’s disease, you are at significant risk. Coeliac disease also runs in families. If you have any symptoms, such as abdominal pain, diarrhoea or unexplained weight loss, see your doctor – early diagnosis and treatment of both Crohn’s and coeliac disease can prevent serious health complications down the road.

If you’re an only child, your chances of being overweight are at least twice as high as someone who has siblings, a 2012 European study discovered. The thinking goes that only children don’t get out and play as much, and are more likely to have TVs in their bedrooms than their peers with siblings.

But research shows that having an overweight or obese older sibling also drives childhood obesity – especially if the sibling is of the same sex.

Surprisingly, sibling influence on weight gain is higher than a parent’s influence.

“There are also lots of other factors that affect obesity, such as what occurs at school, what sorts of foods are available, what the friends are like,” explains Dr Michael Tam, staff specialist in general practice at the University of New South Wales.

A US study also reveals that the eating patterns we develop and share with our siblings during childhood continue to resonate long after we’ve grown up.

Those petty fights over who got the better gift at Christmas or who’s been snooping in your room can have consequences long after the shouting has stopped. A 2012 US study of pairs of teenage siblings found those who squabbled over violations of property or personal space suffered from greater anxiety and lower self-esteem later in life, while tiffs over equality issues were linked with residual depression.

A 2013 study revealed that bullying among siblings causes as much mental distress as being bullied at school. And, according to Stanford School of Medicine in the US, if you have a parent or sibling with major depression, you’re two to three times more likely to develop depression yourself.

When a firstborn child develops autism, their younger siblings have a seven per cent chance of developing the condition as well, according to a 2013 Danish study.

Previous estimates had been a much higher 18 to 19 per cent. “This research shows that the risk of a couple who have a child with autism having another child with autism is significantly lower than previously thought,” says Professor Andrew Whitehouse from the Telethon Kids Institute Australia.

If you got your first period about a year after most of your peers or became sexually active later and have brothers, those boys could be to blame. A girl’s onset of reproductive maturity can be affected if she has an older brother, and when she begins sexual activity may be delayed if she has a younger brother, says an Australian study.

Co-author of the study, Professor Debra Judge, says the later menstruation may be due to an evolutionary imperative where the older brother protects the younger sister. She suggests older sisters often care for little brothers and so don’t have the time for relationships outside the home.

Text: bauersyndication.com.au

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