Colin Craig has been earning himself a few column inches lately.

Somewhere in between announcing that he chose to be heterosexual and his declaration that John Key is “too gay” for his electorate, Craig took time to claim science was on his side in the debate around marriage equality.

Writing in the Waikato Times, Craig leaned on the results of the Human Genome Project in search of support for his claim that sexual orientation is a choice.

In fact, Craig’s comments highlight a common misconception about the way variation in our genes and the environments we grow up in contribute to variation in our bodies, brains and behaviours.

Moreover, Craig’s unstated assumption that a scientific understanding of the origin of homosexuality is relevant to the marriage debate is an example of the way science can be misapplied in politics.

Craig is right that we do not know of any gene variants that, in and of themselves, are enough to make us gay or straight. But that does not amount to saying that genes do not contribute to sexuality.

The process that turned a single cell into the person your are today is immensely complex.

It involved the inter-play of the products of thousands of genes and was influenced by the environment in which those genes were expressed.

There is no doubt that the different genes we start with contribute to human diversity, but finding the effect of a single gene in this elaborate process is rather like pinning down the contribution of particular spice to a complex recipe.

Indeed, one of the outcomes of the Human Genome Project has been the problem of so called “missing heritability” - traits that we know to vary as the result of genetic differences, but for which no particular genetic differences have been isolated.

Sexual orientation is one of these traits. A recent study on comparing identical and non-identical twins found that, among men, something like 39 per cent of variation in sexual orientation can be explained by variation in genes. The remaining variation being explainable by differences in environment.

We should be cautious about extending the results of single study too far, but it certainly seems there is some genetic input to sexuality.

If Craig is wrong in claiming that genes don’t contribute to sexuality he’s even more wrong in assuming the lack of a genetic basis for sexuality would imply that we can choose our orientation.

Traits are not more or less innate, or more or less able to change, simply because they have a genetic basis.

Most neural tube defects are caused by a lack of folic acid in early development, but despite having an environmental basis, they are sadly not reversible.

Craig has talked about environmental contributions to sexuality; making the seemingly contradictory claim that homosexuality is both a choice and the result of abuse during childhood.

So, Craig’s view is not not supported by evidence. But what if it was?

What difference would it make if there really was scientific evidence to suggest that Craig could just as easily have fallen in love with a man as he did his wife?

I heartily endorse Craig’s sentiment that research and evidence should “support us in making good decisions”, but utlimately it’s us that have to make the decisions.

Science does not provide answers to moral and ethical questions, rather, it allows us to understand the implications of our decisions.

Without a reason as to why the causes of homosexuality are relevant to whether we should include or exclude same sex couples from marriage Criag’s comments are not only wrong, but irrelevant.