By Easha Vigneswaran
Impulsivity is defined as the acting without forethought. Neurobiologists have been trying to find out what causes such behaviours in humans and how knowledge of this can help clinicians understand disorders such as delinquency, antisocial behaviour associated with suicide and aggression. Scientists have also furthered the question to find out whether there is a link between the male and female brain in relation to impulsivity.
Broadly speaking, impulsivity is to do with people suffering from the inability to make rational decisions that eventually lead to these types of behaviours. Often these behavioural phenotypes can be caused by neurobiological abnormalities or genetics.1 Neurobiological explanations for impulsivity in humans have been related to hormonal sensitivities. Dopamine and serotonin are neurotransmitters where the former is responsible for reward and pleasure and the latter for inhibition and restraint.2 Abnormal dopamine function in the body has been linked to numerous disorders such as addiction and ADHD, which all relate to issues with impulse control.1 Abnormal serotonin levels have been associated with aggression in certain animal behaviour. A group of scientists analysed serotonin biomarker levels in convicted violent criminals. The biomarkers, cerebrospinal fluid (CFS) and 5-hydroxyindoleacetic acid (5-HIAA) levels were found to be lower in people whose aggressive crimes were committed out of impulse as opposed to premeditation. In rat/mouse models, when 5-HIAA levels were deliberately lowered, these rodents exhibited greater levels of impulsive aggression. An increase in 5-HIAA activity using its precursors showed lower levels of aggression in rodent models.3
Apart from neurobiology, genetics also impacts impulse control in humans. There are numerous genes responsible for the synthesis of dopamine and serotonin neurotransmitters and uncontrolled secretion of these neurotransmitters has been shown to affect impulsivity. The MAOA gene encodes monoamine-oxidase A which metabolises monoamine neurotransmitters (dopamine and serotonin). A MAOA stop codon (C936T) was found to inhibit the functioning of the MAOA gene meaning no MAOA enzyme activity. Without the enzyme activity, monoamine neurotransmitters levels are higher. This discovery was correlated with greater impulsivity observed in males of one Dutch family. Mice who have the MAOA gene knocked out also show greater concentrations of monoamines (as they aren’t metabolised by the enzyme) in the blood and therefore more impulsive behaviour.3
Anatomically speaking, there are key regions of the brain that are linked to impulse control. Studies have suggested that those who are sensation seeking and have increased motor impulsivity tend to have reduced cortical thickness of the brain. This is the region responsible for motor control, emotion and consciousness.4 Scientists have also found a neuropeptide, melanin concentrating hormone (MCH), that is also involved in impulse control. Brain scans revealed a neural pathway used by MCH is located between the hypothalamus and hippocampus, which are regions responsible for emotion and inhibition. When MCH blood concentration was increased in rats, the scientists observed increases in the level of impulsivity.5
The scientific community have also shown an interest in understanding differences in impulse control between men and women. Clinical studies have separated impulsivity into two different types: impulse control and impulse action, of which both have been associated with drug abuse. Using both human and animal models, a variety of responses were observed between men and women dependent on the levels of their sex hormones.6 Many studies imply that men are more impulsive than women. However, recent, more comprehensive studies, suggest that gender differences in impulsive control are more complex. Female smokers show greater impulsivity than male smokers, but men from the control groups showed more impulsivity. Similarly, among female drinkers, women show less restraint than men.7 Overall, the study showed that women who abuse substances tend to show less impulse control comparatively to men who are substance abusers.6
When looking at neural connectivity within the brain, we can also find differences between men and women. Some studies found that men have greater connectivity within the brain hemispheres, whereas women show more connectivity across the hemispheres. This suggests that the male brain shows more evidence of initial perception followed by action, while the female brain shows higher levels of analytical thought coupled with intuition to facilitate action. Due to the development of the brain occurring at a young age, it is difficult to know if these differences are as a result of genetics or whether sex hormone cycles potentially play a greater role.7
Impulsivity in humans is complex and very clearly influenced by many different factors. From neurobiology to genetics, the reasons for what causes impulsive behaviour remain wide ranging and no single factor can decide whether a person exhibits impulsivity or not. By identifying the contributing factors, clinicians can better understand how some individuals may be predisposed or how impulsivity can be influenced from external factors in life. Thus, treatments for these conditions can be discovered to reduce the risk of people falling into criminal, suicidal or antisocial behaviour.
- Pine A, Shiner T, Seymour, B. Dopamine, Time and Impulsivity in Humans. Journal of Neuroscience. 2010; 30 (26) 8888-8896. doi: https://doi.org/10.1523/JNEUROSCI.6028-09.2010
- Weinstein A, Dannon, P. Is Impulsivity a Male Trait Rather than Female Trait? Exploring the Sex Difference in Impulsivity. Curr Behav Neurosci Rep.2015; 2, 9–14. Doi: https://doi.org/10.1007/s40473-015-0031-8
- Bevilacqua L & Goldman D. Genetics of impulsive behaviour. Philosophical Transactions of the Royal Society B: Biological Sciences. 2013; 368 (1615), 20120380. Available from: https://royalsocietypublishing.org/doi/10.1098/rstb.2012.0380.
- Holmes AJ, Hollinstead MO, Roffman JL, Smoller JW, Buckner RL. Individual Differences in Cognitive Control Circuit Anatomy Link Sensation seeking, Impulsivity and Substance Use. Journal of Neuroscience. 2016; 36 (14) 4038-4049. DOI: https://doi.org/10.1523/JNEUROSCI.3206-15.2016/
- Noble EE, Wang Z, Liu CM, Davis EA, Suarez AN, Stein LM, Tsan L, Terrill SJ, Hsu TM, Jung A, Raycraft LM, Hahn JD, Darvas M, Cortella AM, Schier LA, Johnson AW, Hayes MR, Holschneider DP and Kanoski SE. Hypothalamus-hippocampus circuitry regulates impulsivity via melanin-concentrating hormone. Nature Communications. 2019; 10(1) 4923. doi: https://doi.org/10.1038/s41467-019-12895-y
- Weafer J & de Wit H. Sex differences in impulsive action and impulsive choice. Addictive behaviors, 2014; 39(11), 1573–1579. https://doi.org/10.1016/j.addbeh.2013.10.033
- Mitchell MR & Potenza MN. Importance of Sex Differences in Impulse Control and Addictions. Frontiers in Psychiatry. 2015; 6 (24). Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4332159/.