Biology affects behavior also through mechanisms of heredity regulated by genetic principles. The nature versus nurture controversy— that is, how much of our behavior is due to inherited factors and how much to environmental factors—is a question that has plagued scientists for years and is still unresolved.

Genetic principles. Genetics is the study of heredity, the manner in which traits and characteristics (for example, eye color) are passed from parent to offspring. Each human cell, except sex cells, contains 23 pairs of chromosomes, a total of 46. (Sex cells—the sperm and the egg—each contain 23 chromosomes but form a total of 46 when they unite.) Chromosomes are strands of DNA (deoxyribonucleic acid) in the nuclei of cells that carry genetic information, genes.

Genetic studies in psychology. Researchers in the field called behavioral genetics study, through both family and twin studies, the way in which genetic factors affect behavioral traits. In family studies, the focus is on the investigation of blood relatives to see how similar they are with respect to some trait (for example, the occurrence of a mental disorder such as schizophrenia). Twin studies compare identical twins and fraternal twins for various similarities in appearance and behavior to see which traits/behaviors are affected by genetic makeup. In some cases when twins have been adopted into separate families, it is possible to expand the information and determine which traits are affected by environment rather than heredity.

Studies of genetic defects (for example, certain types of developmental disabilities) also provide pertinent information on the effects of heredity/environment upon behavior.

  • Down syndrome is a human genetic defect in which there is an extra 21st chromosome. People with Down syndrome have distinctive physical features and often some type of developmental disability.

  • Phenylketonuria (PKU) is an inherited metabolic disorder. The presence of a particular gene keeps the individual from being able to process the amino acid phenylalanine. An excess of this chemical interferes with the formation of myelin in the brain and can produce a type of developmental disability. The genetic problem can be detected by a PKU test given at birth and can be regulated by dietary methods.