The impact of intestinal flora on health conditions is known, thus fecal transplants for numerous conditions. Last week I wrote about an experiment with mice verifying the impact on obesity.

As a consequence, there are numerous products being marketed as “probiotic.” We have no idea whether those products are helpful, harmful, or simply benign.Bacterial flora are an instance where we know some bacteria are essential, and we know some can be added beneficially, but we do not know which ones to add. There are promising studies with lactobacillus acidophilus shown: We almost know about GI flora, but not quite enough yet.

There are other topics where we are at a similar place. We know a lot about what mental illness is and isn’t. We know that increasing serotonin in the interstitial spaces of the brain helps with depression and that too much serotonin is associated with schizophrenia. As of this point, however, we don’t have imaging or blood tests or biopsies that will tell us who is mentally ill–we use written testing and observation not laboratory tests to diagnosis it. When physicians attempt to treat it, it is largely a matter of trial and error. We do not know beforehand whether a particular selective serotonin re-uptake inhibitor (SSRI), such as fluoxetine or venlafaxine will work with an individual, or if any SSRI will work. Some people will do better with buproprion, which uses a different mechanism. Others will do better with a serotonin–norepinephrine reuptake inhibitor (SNRI) such as duloxetine. Others will experience no clinical effect at all. We almost know about mental illness, but not quite enough yet.

These examples lead to a more general question about what we know and do not know. It is usually phrased as “nature vs. nurture,” but it is really genetics vs. environment. I am not sure if the the “versus” between them is appropriate–something else I do not know–as it is the relative interaction of the two rather than a false choice between them that is a more likely source of the truth about who we really are.

Researchers often conduct identical twin studies, controlling for genetic variation by comparing the environmental impact of twins with nearly identical DNA. However, the studies are limited: it is intriguing if they both smoke the same brand of cigarettes or like the same foods despite very different upbringings, but it does not neatly tell us which behaviors are genetic and which are environmental.

Similarly, we know that 3/4 of children of two bi-polar parents are likely to have bi-polar disorder, which seems to indicate a Mendelian genetic inheritance, but we only almost know about the inheritability of mental illness, not quite enough.

Even where we know that a trait is inherited, we often do not know what genes or constellation of genes are associated with a given, visible trait. We almost know about the human genome, but not quite enough yet.

As scientists or those of a scientific bent, we are obligated to say what we know and what we don’t know, being able to distinguish the difference. It is not always an easy distinction to make, but is central to our effort to know more, and eventually know enough.