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ELEANOR HALL: To some potentially good news for young girls who spend a lot of time with their grandmas.

A study from Cambridge University in the UK has found that in some populations the survival rate of a granddaughter living in the same house as her paternal grandmother is higher than if she were just living with her immediate family.

But the grandmas did not have such a positive effect on their grandsons.

The lead researcher Molly Fox told reporter Barbara Miller that the findings add to the understanding of what's known as the grandmother effect.

MOLLY FOX: What the grandmother hypothesis basically says is that if a grandmother helped provision her grandchildren while they are toddlers and they're weaned but they are still sort of helpless, she can increase her genetic contribution to future generations.

BARBARA MILLER: Why only grandmothers? Why not grandfathers?

MOLLY FOX: Oh, because men can reproduce until the day they die. They can have a baby on their death bed and grandmothers basically have 50 years as, you know, that's how long eggs live. So if you are going to keep on living then it is going to be without making any babies of your own.

BARBARA MILLER: So what does your research add to the understanding of the grandmother hypothesis?

MOLLY FOX: Oh well, we sort of look at this idea that if grandmothers are investing in their grandchildren because they are related to them then we should investigate that relatedness itself and the genetic relatedness between maternal versus paternal grandmothers varies with boys versus girls. So in some, I guess what we add to it, is this idea of favouritism, that there should be some favouritism on the part of grandparents investing in their grandchildren.

BARBARA MILLER: Because paternal grandmothers are more closely related to girls genetically, is that right?

MOLLY FOX: Yes exactly. A paternal grandmother and a girl versus a boy - there is a lot of variation there and the reason that grandmothers have this variation in their genetic relatedness of grandchildren is because of inheritance patterns of the X-chromosome.

BARBARA MILLER: And what did you find?

MOLLY FOX: Well what we found was that the survival of a grandchild in the presence of their grandmother actually varies with the variation in genetic relatedness between them. So in other words the presence of, say, a paternal grandmother living in the house with a girl increases her survivorship significantly but her presence in the house with a grandson, that is a paternal grandmother and a grandson, actually in all seven of the populations we looked at, that increases the boy's likelihood of mortality as opposed to if she weren't there.

BARBARA MILLER: That seems an astounding finding. Why would the presence of a paternal grandmother decrease the likelihood that a boy would survive?

MOLLY FOX: Yeah, it was not our hypothesis that she would have a detrimental effect. We just thought it would be a less positive effect and we certainly didn't look into the mechanism but I could at least make some guesses which might be that having a grandparent who is favouring other grandchildren is only taking away resources or an elderly person bringing pathogens into the home.

I guess if someone in your house is not having a positive effect then sort of the neutral option might be more detrimental.

BARBARA MILLER: So this is bad news for boys who spend time with their paternal grandmothers, is it?

MOLLY FOX: (Laughs) Maybe they should try to overcome my results by making that relationship a little more beneficial.

BARBARA MILLER: But you studied seven populations. Just give us a sense of the geographical and time span of those populations.

MOLLY FOX: Well we looked at a huge variety of different populations. We actually looked at a Japanese population from the 1600 or 1800s; a German population from the 1700-1800s; a British population from the 1700s; a modern Ethiopian population; a modern Gambian population; a modern population from Malawi and an historical population from Canada from the 1600-1700s.

BARBARA MILLER: Did you think it has, the findings though have relevance for example for Australian grandmothers today?

MOLLY FOX: Um, you know the populations that we were looking at are all places where the grandmother is living in the house or at least in close proximity with the grandchildren so these sort of traditional populations.

In Australia I don't believe that there is high incidence of grandmothers living in the home so maybe the boys are safe at the moment.