To Julia, Ana and Menchu

Eduardo Costas, professor of Genetics at UCM.

Society is shocked because in Castro Urdiales, a Cantabrian town of 30,000 inhabitants, a 15-year-old boy, accompanied by his 13-year-old brother, allegedly stabbed his mother, causing her death.

This is an extremely complex issue in which evolutionary genetics goes against our most deeply-rooted moral convictions. According to genetics, it is very rare for a mother to kill her children, since she would be eliminating the possibility of her own genes surviving. The selection goes against a mother killing her children.

For the same reason of sharing genes, it is also a bad idea for a son to kill his mother: if we kill our mother, our own chances of surviving (and passing our genes to the next generation) decrease significantly.

No genetic links

But, although many people believe that an adopted child should be especially grateful to his mother, biological reality tells us the opposite: an adoptive mother has no genetic ties to her adopted children.

Nor does an adopted child have genetic ties to his adoptive mother; he cannot even expect her to have more children who share half of their genes with him. It may seem very harsh, but biologically speaking, an adopted child has no genetic ties to his adoptive mother.

According to statistics, it is much more common for an adoptive mother to kill her children than for a biological mother to do so (in fact, they do it around 11 times more). It is also more common for adopted children to kill their mothers than for a biological child to do so (and they do it around 13 times more). Something similar happens with parents, although a father shares fewer genes with his sons than a mother.

quick thinking

There is another factor to take into account to better understand the drama of Castro Urdiales: it is called the fast thinking. As the Nobel Prize winner demonstrated Daniel Kahneman, is one of the two ways of processing information that the brain has: it does so in a non-reflective way, based on pre-established patterns. These quick thinking patterns are the ones with the greatest heritability.

Therefore, from a genetic perspective, there is a high probability that the minor who allegedly killed his adoptive mother in Castro Urdiales did so as a result of quick thinking. And to a large extent he inherited this type of thinking from his biological parents (whom it is even likely that he would never have met).

This type of quick thinking almost always falls into the so-called “confirmation bias”: as you quickly analyze what happened, you are left with only the ideas that confirm your “a priori” beliefs. And since most of the time we humans use this mode of rapid thinking, at first glance we look a lot like our parents.

And slow thinking

Only on rare occasions is our brain capable of generating a slow thinking and rational, extremely difficult to develop, which consumes a lot of energy and does not provide quick enough responses to day-to-day challenges. That is why it is not used much. It does not fall into confirmation bias. And with this type of thinking it is very difficult to find the answers. But it is the thought that comes closest to objective truth. The essence of thought that makes us human.

This slow thinking gives us an extraordinary privilege: it is as if we found the genie inside the lamp. But he is a very special genius. It sets many difficult conditions for us that we must overcome before granting our wish. And the only wish it grants us is to know the answer to certain questions. Science is based on this type of slow thinking.

If we leave aside quick thoughts, with confirmation bias, we must assume, always from a genetic perspective, that the murdered mother was able to go to extremes to ensure that her adopted children came to accept the thoughts and values ​​that she believed to be correct. Despite her efforts, her biology prevented her adopted children from looking much like her.

The environment also matters

To all these factors, biology adds another that is important to understand these dramas: the family environment. Today there are many types of families: biological, adoption, families resulting from second (or third) couples who provide children…

Those of us who have had to act as non-biological parents (during the last 36 years I have been, for better or worse, the adoptive father of who today are three splendid women) want to cling to the idea that it is the environment that sets the tone and not the genes.

Although we are not united by genetics, we like to believe that we have strong ties to our non-biological families. But… what is true in all this?

It is one of the most studied topics in human genetics. The influence of genes and environment can be measured by comparatively analyzing twins, siblings, and people with no genetic link.

Two twins (who share all their genes) raised in the same family also share a common environment, but they do not share a differential environment (for example, when one of the twins is in one place and his brother is in another). Two brothers (who share half of the genes) raised in the same family also share the common environment, but do not share the differential environment. Two children adopted by the same family, but who are not related to each other, they only share a common environment

Little family influence

After analyzing a huge number of people for more than 70 years in different countries and environments, it has been estimated that the importance of genes in various behavioral characteristics or intelligence is close to 50%. The influence of the differential environment is around 30% and that of the common environment is barely responsible for the remaining 20%.

This means that, in a biological family, the resemblance between parents and children can be very great (50% due to genetics and 20% more due to the shared environment). On the other hand, the similarity between parents and children in an adoptive family is very small (barely 20% of the common environment).

No matter how much adoptive parents try to educate their children, they can only influence about 20% of what they will become.

Some studies go further and detect statistically significant resemblance even in certain ways of thinking of adopted children with their biological parents. Our biology is like this.

adopted child syndrome

In any case, the moral indignation associated with the fact that these boys allegedly killed their adoptive mother, who allowed them to have a much better life, does not make much sense biologically speaking.

Rare events like this occur in the extremely complex world around us. Events for which it is very difficult to give an explanation. Events that you have to think about for months following a rigorous method and abandoning quick thinking.

Lawyers know this well: When they represent adopted people in capital cases, they frequently use a theory known as “adopted child syndrome.”