Sara Stinson is a professor at CUNY Queens College. She received her Ph.D. from University of Michigan in 1978. Her focus is on physical and developmental variations among living human populations. She studies environmental influences on growth and the evolution of body size. Her research has been mostly on South American populations, including variation in indigenous South American populations, effects of high altitude hypoxia, and growth of lowland tropical forest populations.
Growth Variation: Biological and Cultural Factors
Variation in size is one of the most obvious ways that humans differ. These differences are also easy to measure and compare across populations, making this a well-studied topic in biology/anthropology. Many factors can contribute to growth variation across and among populations such as genetic factors, nutrition, environmental conditions, social conditions, and cultural conditions.
Genotype can have a strong effect on growth as shown in the next two examples. Our ancestry and previous natural selection pressures on our ancestors affect growth patterns.
The range of the average height for the tallest and shortest populations is less than one foot. The shortest population, the Efe pygmies of central Africa, is less than 5 ft. tall on average. While poor nutrition plays a role, it has been shown that these populations have a reduced number of sites for growth hormone to attach on their cells which would lower the effect the hormone has on them. Smaller body size could have been selected for since it would be advantageous to be small in this hot, humid environment, but no genes that could be attributed to natural selection have been identified.
Another genetic example of growth variation is the difference in males and females. In the US male and female average height is approximately 6 inches different. The height ranges can be represented by two overlapping bell curves. While males tend to be taller, females tend to carry more subcutaneous fat than males, and females carry their weight in different areas than men. These sexually dimorphic tendencies exist in every human population, and are contributed to by hormonal differences during adolescence and puberty.
Most size variation in the world’s populations is attributed to environmental factors.
Socioeconomic Status is one of the major players that can contribute to size variation. Socioeconomic status can limit access to quality health care and nutritious food, while having an increased exposure to disease. Scientists can look at income, education, occupation, and household possessions to determine this status. In almost EVERY study done so far children from high-income families are taller than children from low-income families. This difference is more noticeable in developing countries than in industrial countries. Economic inequality exasperates this phenomenon. Some argue that short stature due to economic status could be attributed to higher levels of stress in low-income families. Low-income is also associated with higher rates of obesity in developing and industrial societies where high-caloric, low-nutrient foods are cheapest.
Ideology and Beliefs have also been shown to influence growth. Families in Niger would rather keep their possessions important for determining rank, instead of selling them for money, because in this culture status is very important. In the US parents may opt out of forcing their children to eat healthy foods in order to maintain a peaceful dinner environment because here an emphasis is placed on having positive family dinners. In Tanzania children from monogamous marriages were taller and thinner than children from polygamous marriages.
Environmental Pollutant (toxicant) exposure has been associated with reduced height. Prenatal exposure to cigarette smoke and/or alcohol shows babies that are born smaller. Children with high blood lead levels have slower prenatal and postnatal growth and delayed puberty. Polychlorinated biphenyls (PCBs) can increase maturation rates in girls. Girls with higher levels of PCBs have earlier menarche, but there is little affect on overall growth patterns.
Recent Trends in growth patterns show that height and weight have increased in populations around the world. Over the last 100 years in Sweden, female height has increased by almost 5 inches and male height by almost 8 inches. This next part is a little unintuitive, but trends show that we are maturing earlier as well as faster. The age of menarche has decreased from 15-16 in the 19th century to 12-13 years old in most industrialized societies. Increases in leg length are higher than the increase in trunk length. European populations are now on average taller than Americans. While the height of populations in Oaxaca, Mexico has remained the same for 500-3000 years, during the last 30 they got a health center and secular trends have showed up in children’s growth patterns. Recent Mayan immigrants to the US have children that are approximately 4 inches taller. 70% of this height increase is attributed to longer legs.
Why are these trends happening though? The change in the last 150 years has happened too fast for it to be attributed to genetics. The reduction of infectious diseases could be a factor-improvement of world populations overall. Greater gene flow between populations could also be a factor. The increase in body size appears to be slowing and plateauing in affluent countries. A downside to these trends is that an earlier age of menarche has been deemed risk factor for breast cancer, heart disease, and type 2 diabetes. Obesity is becoming an ever-increasing problem around the world. About 67% of US adults are overweight or obese.
Nutrition and Disease throughout the Life Cycle
Parental Growth usually assessed by birth weight can give indications on how an individual will grow. Babies less that 5.5 lbs are considered to have a “low birth weight” which can result from prematurity, slow growth during gestation, genetics, or multiple births. During WWII women in Russia and the Netherlands were under siege from the Germans reducing the amount of food entering the cities. Birth weight decreased in these populations, with 50% of babies in St. Petersburg being born with low birth weight. An increase in maternal nutrition during pregnancy shows an increase in birth weight. However, if a woman has been malnourished her entire life, increasing nutrition during pregnancy is not enough to increase birth weight. Babies can adjust their metabolism to what they (and their mother) are experiencing during pregnancy. So if a woman is normally well nourished, but during pregnancy is malnourished, this cannot only make the baby be smaller, but also change the child’s overall metabolism. Maternal cues to the fetus including placental transfer of nutrients or hormones would be an epigenetic effect. Studies have shown that a women’s birth weight is a strong predictor of her children’s birth weight.
Infancy and Childhood Growth is more important, however, in terms of its affect on body size and health. Disruptions in the growth of children are the main cause of small adult body size. Human milk is relatively low in fat and protein content indicating frequent nursing and slow rates of postnatal growth. Breast-feeding infants is very beneficial for developing infants. Other feeding methods do not provide anti-infective agents that help in the development of the baby’s immune system. These agents include antibodies against specific bacteria and viruses; white blood cells; and proteins, fats, and carbohydrates that convey antimicrobial action. Bottle-fed infants have higher rates of illness and mortality than breast-fed infants. It can be “argued that breast milk is the nutritionally ideal food for infants because it is the food that has evolved to meet their needs. “ Breast-feeding is associated with lower risk of ear, respiratory and gastrointestinal infections; sudden infant death syndrome; asthma; and reduced risk of obesity and diabetes in the children.
During the first few months of development, infant growth rates are consistent in poor and developed countries. However after the age of 4-6 months the growth rate in infants in disadvantaged circumstances decreases. By two years, there is a substantial different in height. The slowing of growth is referred to as growth faltering. By 5 years old, children in poor countries fall below the 25th percentile for height. In rural Mali cultures young children are not encouraged to eat, and must fend for themselves. But in rural highland Peru children are given preferential treatment, and in times of food shortage are given food over adults. Growth faltering is not only nutritional but also caused by diseases. Children and infants exposed to pathogens may experience more growth faltering.
What did you think of the Mali cultures?
Adolescent Growth is less affected by environmental factors due to the fact that this period is characterized by slower growth rates. In some environments rapid maturation is necessary due to high mortality rates. Rapid maturation results in smaller adult body sized, but the individuals can reproduce at an earlier age. In these cultures there is a trade off between rapid maturation and small adult stature.
Catch-up Growth is a period of faster growth that follows a disruption. Turkana populations are initially below the 50th percentile and sometimes lower than the 5th percentile in height, but when they reach maturation they are on the normal standard. This can be explained by a longer growth period in these populations.
Is small body size an adaptation? It has been argued that when a human is exposed to low-nutrition environments they may cut their losses and remain smaller so that the resources that they have can go father. This would allow for more fit individuals that are smaller than their maximum potential.
Human height variation is a complex outcome of gene and environmental interactions. Narrowing down the cause of height to one factor is impossible, and when looking at height distributions you must take into account genetic history, environment, nutrition during development, social status, and cultural influences.