The Human Spark

The science of human development

How Do We Know?

There are four sources of knowledge.

The sense that one knows a fact to be true has four different foundations and, therefore, four meanings depending on whether the understanding originated in direct sensory experience, inference, logic, or the spoken or written statements of others. We know rain will wet our clothes because we have experienced this event. This knowledge need not be conscious. Most adults do not know why the most aesthetic sculptures of the human body are those in which the length of the head and chest is about two-thirds of the total height of the statue.

Everyone knows, albeit with slightly less certainty, that a person laughing heartily is probably feeling happy, a moist earth in the morning implies that it rained during the night, and thick, gray clouds on a cold day are a probable sign of snow. All children begin to make inferences about themselves and the minds of others by their second birthday. They automatically help an adult in need of assistance in retrieving an object, even if the adult does not explicitly ask for help. Humans are the only species that automatically infers invisible processes as the presumed cause of an event.

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The ability to deduce logically correct conclusions and to detect illogical ones is a third source of knowledge that involves sentences or mathematical equations. The original positing of the Higgs particle, many years before physicists claimed in 2013 that they had found it, is a classic example. The retired British philosopher Anthony Flew had been a committed atheist until late in life when he examined the logic of the natural scientist’s explanation of the origin of the universe and life on earth. Finding the logic of these arguments unpersuasive, he concluded that positing a God was no less logical than denying a transcendental being.

Psychologists who study humans often rely on the fourth source of knowledge- spoken and written statements. It has proven difficult to persuade social scientists that there is often a minimal relation between a person’s statements about themselves or others and the evidence that comes from direct observation. Thus, the meaning of conclusions based only on questionnaire replies has a specific and restricted meaning. Students who were told by an authority figure that objective measures indicated they slept very well during the prior night, even when they had not, performed better on cognitive tests than those who were told their sleep was poor. The students’ verbal reports of how well they slept had no effect on their performance. This observation points to the ease with which an authority can persuade a person of a fact that is not true.

Because the meaning and validity of every conclusion depends on the source of evidence it is critical that investigators and the public reflect on the basis for a presumed fact. Each source of evidence is like a lens with a fixed curvature. A viewer can polish the lens to render perception a little clearer, but each lens can reveal only a limited set of events. A curtain punctuated with many tiny holes separates scientists from the phenomena they want to understand. The view from a single hole, analogous to reliance on one source of evidence, cannot provide the information needed to satisfy this wish.

The habit of gathering one measure of a possible causal condition or outcome, as if nature preferred silver bullets, has made it difficult to accumulate a large number of reliable psychological facts. Single conditions, such as poverty, day care attendance, security of attachment, maltreatment, being bullied, divorce, or maternal depression, independent of the person’s gender, social class, ethnicity, culture, temperament, diet, and illness history, are rarely the cause of any specific outcome. A pattern of four conditions was needed to predict the likelihood of adolescent suicide in the United States: growing up in a disadvantaged family, living in a rural area of a Western state, and a Monday during the warmer months from April through September. A combination of four properties protected a small number of infants growing up on the Hawaiian island of Kauai who experienced a traumatic birth or severe stress during the first two years from developing a problem during adolescence. These youth were first-born, had no more than one younger sibling, a younger mother, and possessed an easy infant temperament.

It is easy to arrive at incorrect conclusions when regression equations are used to remove the contributions of conditions that happen to be essential to an outcome. One pair of social scientists interested in the relation between life satisfaction and place of residence used statistics to remove the important contributions of income, age, gender, ethnicity, years of education, and employment in a sample of 1.3 million Americans. Because these six conditions make important contributions to life satisfaction the analysis revealed that the happiest Americans lived in Louisiana and the least happy adults lived in New York state. This counter-intuitive result rubs against the fact that more Americans prefer to live in New York rather than Louisiana, as well as the results of a Gallup poll which found that the residents of Louisiana are among least happy Americans. No biologist would use statistics to remove the contribution of rainfall to the growth of rose bushes in order to estimate the independent contribution of sunshine because roses need both to grow. Most psychological outcomes, and especially the symptoms of mental illness, require the combined action of several factors, biological as well as experiential. 

There are few silver bullet outcomes. The popular practice of relying on one measure to prove the correctness of a prediction, whether answers on a questionnaire, blood flow, or cortisol level, ignores the fact that almost every outcome measure can be the product of more than one set of conditions. A given judgment of subjective well-being, for example, is due to a variety of conditions, including culture, social class, health, and age. Discovering why adults vary in their judgment of well-being is a puzzling phenomenon that invites serious study. The reply, “I am very satisfied with my life at the moment,” given by a middle-age, black woman from Alabama whose annual income of $30,000 is earned as a helper at a nursing home, and by a white, 25 year-old female French lawyer earning more than $100,00, are not answers to any important question. The task is to understand the various conditions that might generate these replies.

What are we to do? Decisions must be made by individuals and government officials who need facts that are usually uncertain. It is difficult for the average person to understand the scientific evidence behind many conclusions. As a result, valid conclusions do not always change minds. Facts, reason, and community sentiment join in determining the beliefs that a majority in a society will regard as a persuasive reason for action.

Jerome Kagan, Ph.D., is an emeritus professor of psychology at Harvard University and one of the pioneers of the field of developmental psychology. His latest book is The Human Spark.

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