A Swedish study of nearly 71,000 adopted people has used data from both their natural and adoptive parents to find that family history is a greater risk factor than lifestyle for developing breast, prostate, or colorectal cancer.

The genetic factors behind these three major cancers are well established, but this large survey of data published in the European Journal of Cancer has looked a unique group of people in a bid to disentangle familial risk from environmental factors.

By looking at adoptees, the researchers were able to see how being raised in an environment that was independent of hereditary genetics - being brought up by adoptive parents - revealed the true extent of the biological influence exerted by the genes of their real parents.

In effect, the study separated out the cancer effects of family history versus those of the environment.

If a person's biological parent had cancer, there was an 80-100% higher chance of them developing the same disease than if their natural parents were free of the cancer.

Yet the history of adoptive parents had no influence on the risk of developing cancer. If an adoptive parent had cancer, there was no increased effect on the adoptee.

The study was led by Bengt Zoller, a reader at Lund University in Sweden. Dr. Zoller says:

"The results of our study do not mean that an individual's lifestyle is not important for the individual's risk of developing cancer, but it suggests that the risk for the three most common types of cancer is dependent to a greater extent on genetics."

The study also found that adoptees who had a biological parent with cancer developed the disease at a younger age than those without a biological parent with the same cancer. This effect was not seen in relation to adoptive parents, however - whose cancer had no influence on the adoptees' age of disease onset.

Robust cancer data

The researchers analyzed data held in the Swedish Cancer Register, the validity and coverage of which is "almost 100%," and in the Swedish Total Population Register, a "unique" block of information that also has "very few missing data."

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If a person's biological parent had cancer, there was an 80-100% higher chance of them developing the same disease than if their natural parents were free of the cancer.

The fact that adoptees do not share the same family environment as their biological parents means that studying these data gives a purer picture of the genetic influence on cancer.

Even studies of twins, who share the same inherited genes, cannot separate the influence of parental genetics versus the influence of the environment they create.

Talking to Medical News Today, Dr. Zoller explains this further: "I think the study design is convincing regarding the possibility to separate nature and nurture. The classical study design for this is otherwise twin studies. Monozygotes are genetically identical but share environment to the same degree as dizygote twins (who share 50% genetic similarity)."

Dr. Zoller also told MNT that most adoptive children in Sweden are adopted before the age of 1, meaning their environment is separate from that of their biological parents. "We controlled for age, time period, sex, region of residence, and education."

"Although it is well known that genetic factors contribute to the risk of prostate, breast and colorectal cancer," the study paper says, "our finding that genetic factors seem to be more important than family environmental factors in the familial transmission of prostate, breast and colorectal is novel."

The study analyzed the records of 70,965 adoptees born between 1932 and 1969. Their cancer diagnoses were made during the period between 1958 and 2010. Prostate cancer cases numbered 798, breast cancer cases were 1,230, and colorectal cancer, 512.

The researchers conclude that their "novel findings" are useful in the clinic for doctors assessing individuals' risks of cancer as influence by the history of their biological parents. Dr. Zoller adds:

"The occurrence of breast cancer, prostate cancer and colorectal cancer in biological parents is an important risk factor that should be included in patients' medical history and examinations.

It is therefore important that doctors ask about family history so that they can decide whether further tests are needed."

In other cancer genetics research, a study published in July 2013 found that family cancer risk may be wider than relative's specific type - that is, the higher risk of cancer was not confined to the specific type carried by the parent, and the risk of other cancers was raised, too.

Meanwhile, parental cancer hands down other effects - including the inflation of perceived prostate cancer risk by increased diagnosis seeking, the conclusion of Swedish research in 2010.

Another genetic analysis has found that family history of colorectal cancer may increase the risk of the aggressive form of the disease.