How Much of Ourselves Are We Born With?

Today is Universal Children's Day, which was established in 1954 to promote international togetherness and awareness among children worldwide. In today's article we are revisiting the nature versus nurture debate.

The phrase ‘nature versus nurture’ is derived from early studies on the effects of parenting on childhood development. Researchers sought to determine the relative contributions of an individual’s innate qualities, determined by one’s genes (nature), and parenting or personal experiences (nurture) on the psychological and behavioral traits of children. If a child shows aggressive behavior, was he or she genetically ‘programmed’ to behave in such a way, or is it the product of his or her upbringing or environment? Consider if one of the child’s parents is also aggressive. Did the child acquire this behavioral trait through exposure to his or her parent's behavior or through inheritance?
No concept is as pervasive in the study of health and disease as distinguishing the effects of internal and external stimuli on bodily function. Since Claude Bernard elegantly introduced the idea, it has not only formed the basis of modern physiology but has also helped us understand numerous pathological states in terms of the interaction between inherited and environmental factors.
Over the years, the controversy has extended beyond childhood behavior to intelligence, sexual preference, and the propensity for certain diseases (see ''The Origin of Intelligence'' and ''Can You Raise Your Kids Gay?''). Despite being heavily researched, at least two problems make the nature versus nature debate a major challenge facing modern biology. The diseases and traits being investigated, particularly those to do with the brain, are themselves complex and often hard to characterize. Moreover, as our understanding of biology progresses, separating the consequences of intrinsic and extrinsic factors on a certain physiological or pathological state becomes increasingly difficult (see here).

Shedding Light on the Issue Using Twin Studies
Separating the effects of genes and environment on childhood development and the pathogenesis of diseases can be achieved by performing adoption and twin studies. Francis Galton first proposed this approach in 1875. It gained impetus at the beginning of the 20^th century when Gregor Mendel’s insights into the mechanism of heredity became widely known. Identifying differences in specific traits by studying monozygotic twins, who share identical genetic information, over portions of their lives gives us insights into the contribution of the environment in developing these characteristics.
In terms of psychological traits, separated twins usually grow up to be very similar even when brought up in substantially different environments. In the landmark Minnesota Twin Study, which began in 1979, researchers studied more than 100 sets of twins or triplets that had been separated in infancy and raised apart from one another. They found that genetics can explain up to 70% of the variability in personality, intelligence, and temperament between the twins [1].

Implications Beyond Medicine
Investigating the relative contributions of innate and acquired factors in human psychology and health can have far-reaching consequences. Not surprisingly, the nature versus nurture debate has also made its way to the courtrooms. Many experts believe that criminality, for example, is a trait that is predominantly inherited. Thus, defense lawyers sometimes argue (with varying degrees of success) that, in certain cases, people accused of committing crimes cannot be held responsible for their actions because they cannot be held accountable for their DNA.
The discovery of rare mutations that strongly predispose to aggression, such as that of the monoamine oxidase type A gene, has helped encourage the acceptance of such legal arguments [2] (see also ''Tracing the Roots of Aggression"). Biologists and physicians often oppose legal battles that attempt to make use of such a defense. They believe that the public often poorly understands the link between genes and behavior, which is a complex issue.

Is the Debate Obsolete?
Over the past few decades, we have made some astounding discoveries regarding how our genetic material is controlled. We now know that DNA is not the rigid, unchanging blueprint of our entire lives that it was once thought to be. Gene expression is a flexible (yet tightly regulated) process that is modulated continuously in health and disease.
Epigenetics (meaning ‘in addition to’ genetics) is the field of biology that deals with the alterations in gene expression that occur in the absence of changes to the DNA sequence. These changes can persist over long periods of time and, perhaps most interestingly, can be inherited from one generation to the other. The signals that trigger epigenetic changes can come from within the organism itself or from the surroundings (see "Lamarck's Last Laugh" on p.4).
The more we learn about epigenetics, the smaller the distinction between nature and nurture becomes. Consider an example. Exposing an individual to stress can alter the expression of proteins involved in the pathogenesis of mood disorders [3]. This altered expression can persist not only throughout the individual’s lifetime, but can also be transmitted to his or her offspring. If one of this individual’s children eventually develops depression, is the contribution of the exposure of the child’s parent to stress inherited or environmental? The simple answer is both. It thus comes as no surprise that, nowadays, many experts consider the debate obsolete.

Although we should not insist on drawing a line where boundaries are becoming less and less clear, making a distinction between the effects of nature and nurture aids our understanding of complex biological processes.

[1] Bouchard TJ Jr et al, Science, 1990
[2] Brunner, Nelen et al, Am J Hum Genet, 1993
[3] Murgatroyd, Nat Neurosci, 2009

by Ahmed Khalil
This article originally appeared in CNS Volume 7, Issue 3, Nature vs Nurture
 

The Kaspar-Hauser Syndrome - What Child Neglect Tells Us About Nature and Nurture

Nuremberg, Germany, 1828: A savage and mysterious adolescent boy named Kaspar Hauser appears in the city. He has little knowledge of language, and both his gait and behavior are reminiscent of a young child [1]. Very likely, he was imprisoned throughout his childhood, and raised under conditions of extreme deprivation. Kaspar Hauser becomes a famous attraction of the 19^th century, scientists and philosophers worldwide study his behavior attempting to uncover his origin. Some think that he is the prince of Baden, who was abducted from his cradle; others claim he is an imposter.

Whatever the truth of Hauser’s life, his case is the oldest documented and most famous case of severe childhood deprivation. Extreme hospitalism, or neglect-mediated failure of development, is thus also called “Kaspar-Hauser Syndrome”. This malady is characterized by retarded physical development, perceptual-motor skills and language as well as extreme anxiety. It is caused by sensory deprivation and lack of social contact or abuse during early childhood.

LACK OF SENSORY INPUT IMPAIRS NEURODEVELOPMENT

 

Another genre of child neglect is the wolf child or feral child, who has lived isolated from human contact and has no experience of  human behavior and language. The most famous example is Mowgly from the fictional story The Jungle Book. The difference between Kaspar-Hauser Syndrome children and feral children can be seen in their behaviour. Feral children behave like the animals with which they lived (e.g.wolfes), because they have learned their behaviour instead of human behaviour. Severely deprived Children do not learn any behaviour and are thus even more retarded in their perceptual-motor skills and development.

Abnormal brain development following sensory neglect in early childhood. Left: CT-scan of a healthy three-year old with an average head size. Right: CT-scan of a three-year old child suffering from severe sensory-deprivation neglect; this child’s brain is significantly smaller and has enlarged ventricles and cortical atrophy.

Studies of child neglect
As it would be brutal to deprive children of care for the sake of science, most studies rely on case reports of severely neglected children [2]. Thereby, often, the child's medical and social history is unknown. As in the case of Kaspar Hauser, scientists usually do not know the exact form of neglect, or whether neurological defects are a direct consequence. Many studies on hospitalism are based on children raised in badly-run orphanages [3]. Children who were adopted from Romanian orphanages in the early 1990s were often considered to be extremely neglected. Studies with them show that early life nurturing is critical for neurodevelopment. If nurturing is absent for the first three years of life before adoption into a social environment, newly made positive experiences may not be sufficient to overcome the malorganization of the neural system.

Principles of neurodevelopment
To understand how deprivation impairs behavioral and cognitive capacities, we need to first understand the principles of neurodevelopment: At birth most neurons are already present, yet they have to organize into functional systems. Thereby activation of recurrent patterns  appears to be an important factor. Neurons that are not sufficiently activated undergo apoptosis in a ‘use it or lose it’ manner. The sculpting and refinement of neural connections is resolved by dendrite and axon sprouting, followed by synaptogenesis. The latter process is tightly regulated by growth factors and adhesion molecules, whose expression is regulated by environmental cues mediated by the senses.
The brain develops in a sequential fashion; different areas form, organize, and become fully functional at different times during childhood. There are thus different sensitive periods for each brain area and neuronal function. Disruption of critical cues can alter the developmental processes and lead to diminished capabilities in the neural system for a given sensory modality. Disruption of critical neurodevelopmental cues can result from (1) lack of sensory experience during sensitive periods (e.g., neglect) or (2) abnormal patterns of necessary cues due to extremes of experience (e.g., abuse).

Clinical impacts of child neglect
Disturbed neurodevelopment results in defects in language, motor delays, impulsivity, disorganized attachment, dysphoria, attention deficits, and hyperactivity. Furthermore neglected children have a reduced frontal-occipital circumference, which is an estimate of brain size in young children. Furthermore, CT scans from neglected children show enlarged ventricles or cortical atrophy [4]. MRI studies with maltreated children also demonstrate decreased metabolic activity in a number of brain areas [5].

EARLY LIFE NURTURING IS CRITICAL FOR NEURODEVELOPMENT

 

The severity of neurological issues increases with the time children spend in an adverse environment: The earlier and more pervasive the neglect, the more indelible the deficits. After the children are removed from the neglectful environment, the degree of recovery is inversely proportional to age in which the child was removed from the neglecting caregivers [4].
We learn from these studies that neurodevelopment depends on both nature and nurture. Many functions of the brain result from a complex interplay between genetic potential and appropriately timed experiences. Child neglect - defined as the absence of input in critical periods of development - leads to abnormalities in cognitive, emotional, behavioral, and social functioning. Healthy neurodevelopment depends upon attentive nurturing during infancy.
To make sure stories like Kaspar Hauser’s and similar horrible stories of child neglect don't happen again, social services provide help for families where neglect is suspected. If parents are unable to meet their children's needs then professionals intervene to safeguard the child's welfare. As stimulation of a child's senses in the early infancy is so important, it is neccessary to act early if child neglect or abuse is suspected. No issue cries out for a more immediate and all-encompassing response than ensuring the safety and well-being of our children – our hope for the future.

[1] Feuerbach, 1835 (Klett, 1963)
[2] Spitz, Psychoanal Study Child, 1945
[3] Barth Spiegel, 1990
[4] Perry, Brain and Mind, 2002
[5] Chughani et al, Neuroimage, 2001

by Claudia Willmes, PhD alumni AG Eickholt / AG Schmitz

This article originally appeared in CNS Volume 7, Issue 3, Nature vs Nurture