One stifling day at the beginning of the long, hot summer of 2018, I sat in the waiting room at my GP’s surgery. Outside it was dazzlingly bright, but inside the fluorescent lights were still humming. A buoyant doctor strode out and called my name. I took hold of my two-year-old son’s hand and we followed her down the corridor into a small room where she took a sample of my blood. The vial contained thousands of white blood cells. Hidden inside each one was my DNA, the 3.2bn-lettered code unique to every human being that is the blueprint for life.

My son and I were at hospital because my father had been diagnosed with haemochromatosis, an inherited condition in which iron levels slowly build up in the body. Eventually the excess iron begins to damage internal organs and, if left untreated, it can lead to heart disease, diabetes and cirrhosis of the liver. Thankfully, in my father’s case, the organ damage was not too far advanced, but because the condition had gone undiagnosed for decades he now has to undergo weekly bloodlettings. This treatment, while intrusive, means he is otherwise in good health. A happy outcome for him and those of us who love him.

Since the disease is genetic, the NHS offers a test to other family members – that means me, my sister, cousins and, potentially, our children. It’s a simple blood test and the result comes back quickly. If we tested positive, we would need to reduce our consumption of iron-rich food and the levels of iron in our blood would need to be carefully monitored. On the face of it arranging the test should have been straightforward. It wasn’t urgent, but it couldn’t be put off for ever.

I am a neuroscientist and have been fascinated by the idea of biological determinism all my working life, but I struggled with this decision far more than I thought I would. I practised detachment, reminded myself that I believe knowledge is power and that understanding my own body is the most empowering knowledge of all. But still I didn’t book the appointment. I knew that if I tested positive, I would feel compelled to read all the scientific literature to come up with a plan for changing my lifestyle. Would it feel like an extra anxiety-making responsibility or would it empower me to make changes?

I felt relief, but what if the condition affected my son?

I found my opinion changing day by day. In the end the decision was simplified by the GP informing me that the NHS would not consider analysing my son’s blood unless I came back with a positive result myself. I went for the test to discover the risks for myself, and potentially also my son. But it took me weeks to pick up the result. I was surprised by how challenging I found it to step confidently into a position of knowledge when it concerned something so intimate, especially where I would then have to decide for my child. It felt unsettling.

In the end, the results showed “a heterozygous genetic variation”, which means I’m a carrier but unlikely to develop symptoms. I hadn’t anticipated this scenario and, though I felt relief for myself, was frustrated at the lack of definitive peace of mind about my son. He, too, might be affected by the condition in the future but, given my result, the test would not be offered to him on the NHS unless and until he showed symptoms. The whole episode was a lesson in the emotional nuances of what I had previously thought of as a straightforward practical question, and a springboard from which to pursue my investigation into the extent to which any one of us is free to determine our fate. I have been humbled by my sense of our brush with an implacable force.

Since the dawn of humanity our species has been trying to figure out what, or who, is calling the shots. The question of whether we can determine our life’s course or must accept it is largely beyond our control is well up there on our list of thorny conundrums to resolve. Are we fully conscious agents possessed of free will or pre-programmed machines, running on deep drives we are not even aware of?

As far as many ancient cultures were concerned, fate or destiny was all-powerful. The ancient Greeks believed that even the gods could not escape it. These days, at least in Western post-industrial societies, the secular majority of us assume we are the authors of our own life story. We may still speak of someone being “destined for greatness” or “fated to fall in love”, but to our modern minds fate is nothing more than a figure of speech.

Even though we operate from day to day as if our conscious mind was the undisputed captain of a quiet ship, we know that it isn’t that simple. The mind is a much wilder place than that, where conscious decision-making is only a fraction of the full story. In fact, a great deal of decision-making and routine judgment takes place without us being aware of it. If it didn’t, we would scarcely be able to function. Consciously instructing every decision and evaluating every situation would keep us flailing around, trying to get out the front door long after we were supposed to be at our desks. Most of us accept that as well as powerful subconscious forces, external factors shape and, to some extent, determine our lives.

Neuroscience is presenting us with the opportunity to understand more about how we “make our own luck” through exercising choices about which environments or people to cultivate. Those choices are informed by loops of interaction between previous experiences and our perception of the world. At the nub of it all sits our brain, the physical matter we were born with, without which there would be no perception, no memory, no mind.

Most of our decision-making takes place without us being aware

My own fascination with the endlessly interesting human brain came out of working with people with psychiatric disorders. I was intrigued by the question of resilience: why some people move on from a seriously negative life event while others struggle to recover.

During the late 1990s I was a nursing assistant at one of the UK’s leading psychiatric hospitals, where I worked with children aged 12 to 18 who had been detained under the Mental Health Act. They were sectioned and placed in a secure institution to protect them and others. Most had been sent there from across the UK after numerous failed attempts by their local health authorities to support them.

The majority had experienced abuse or neglect early in life. They were extremely vulnerable to peer pressure and found it difficult to lead a healthy, happy life in the outside world. Their destructive behaviours included self-harm, drug abuse and hurting others. I worked in the hospital on and off for three years, before studying biology at university.

I have many positive memories of the place. I remember patients playing basketball in the courtyard, enthusiastically playing bongos during music sessions, playing hopscotch in the corridors or quietly reading Harry Potter novels in their bedrooms. But my overriding memory is a feeling of claustrophobia and frustration for the children. I tried, along with the rest of their therapeutic team, to help them but in most cases I saw little improvement. The experience created a deep desire to contribute to the search for more effective help.

It also left me with questions about what makes us… well, us. Many of the staff had similar upbringings and challenges, but were able to go home after their 13-hour shifts, unlike the sectioned patients. Why was that? What were the underlying differences that produced such divergence in life’s trajectory? Could anything be done to help bulk up a person’s self-protective abilities so that they could flourish no matter what life threw at them?

After my undergraduate degree in biology I went on to do a PhD in neuropsychiatry at Cambridge, where I joined a growing body of researchers working to understand the nuts and bolts of what makes us think and behave as we do. I wanted to bring together, in my book, what I have learned about the factors that shape our behaviours and direct our life’s outcomes.

Knowledge is power, as I had to remind myself when I was feeling jittery before phoning for those test results. The more we understand how our brains, bodies and the environment work together, the more each of us can contribute to the neuroscientific revolution under way.

The Science of Fate by Hannah Critchlow is published by Hodder & Stoughton at £20. To order a copy for £17.60, go to guardianbookshop.com