Much intelligence research has focused on trying to explain the causes of intelligence; where intelligence comes from. At various points in history, particular psychological theorists have suggested that intelligence is primarily an inherited quality (e.g., something formed by biological and genetic forces, and inherited from one's parents) or, instead, primarily something influenced by children's environment (e.g., something influenced by school and parental teachings and by exposure to life experiences and opportunities). Both of these views have merit, as it turns out. Currently, most researchers agree that a combination of both genetic and environmental factors contribute to the development of intelligence.
Genetic Influences on Intelligence
One of the ways that researchers can determine whether intelligence is influenced by genetics is to generate "heritability estimates." Heritability estimates are a mathematical way of representing the extent to which genetics contribute to individual differences in observed behavior such as IQ test scores. Heritability is represented as a numerical proportion that ranges from 0.0 (a case where genes do not contribute at all to differences) to 1.0 (a case where genes explain all of the observed differences between IQ scores). A heritability estimate of .40 would suggests that on average, about 40% of observable differences on a particular trait are caused by genetics. Often, heritability estimates are generated based on studies of identical twins who share identical genetics but who have experienced different environments and opportunities while growing up.
Based on such studies of identical twins' IQ scores, researchers have determined that the heritability of intelligence is approximately .50. This heritability value suggests that about half of intelligence is more or less determined or caused by a child's genetics and biology. The other half is determined by environmental factors which include children's socioeconomic status (a measure of family wealth and social status), parent and caregiver attitudes towards education (whether they believe it is important), cultural and educational opportunities, and other similar social factors.
Cultural Influences on Intelligence
IQ scores are influenced by specific cultural experiences, such as exposure to certain language customs and knowledge from an early age. For instance, many low-income African American children are raised with a language style which may be characterized by an emphasis on story telling and the recounting of personal experiences (e.g., "Did you hear about what Mrs. Smith did this morning?"). Many questions appear in this conversational style, but a fair portion of the time, they function more as rhetorical devices designed to engage a conversation rather than as specific requests for precise information. This style of questioning encourages social bonding but it is not particularly good preparation for traditional intelligence tests which typically demand that children generate a specific single correct response to an examiner's questions. Being used to thinking and responding differently in their day to day lives versus what is demanded of them during testing, such children can end up scoring lower on IQ tests than equally-intelligent peers simply because the testing situation itself does not well fit their life preparation.
In contrast, caregivers in middle income (and above) white and Asian families tend to spend a fair amount of time asking children specific knowledge-based questions that have a single "correct" answer (e.g., What is this word? What shape is a stop sign?). These knowledge-based questions are more like the questions that are used in traditional IQ tests. Therefore, these children may feel more comfortable dealing these types of tasks, and therefore score maximally well (in relationship to their actual intelligence) on such tests.
Socioeconomic Influences on Intelligence
Family socioeconomic status (SES) also affects children's development of intelligence. Specifically, research suggests that children from low SES families tend to have lower IQ scores.
Socioeconomic status has to do with a family's economic and social hierarchy status. Higher SES families have higher family incomes and greater access to necessary and optional but desirable resources than lower SES families. Correspondingly, it is much easier for higher SES families to provide for children's needs in terms of food, clothing, shelter and health care, and to offer them an enriched array of opportunities such as education and daycare than for lower SES families.
Children who feel safe, well-fed, and rested, who are healthy, and whose parents value their intellectual development will be better able and motivated to concentrate their energy and attention on mental tasks and tests. In contrast, children who constantly feel afraid for their safety; who are hungry, sick, or chronically exhausted; and whose parents are overwhelmed and not focused on children's education will simply not have as much energy or motivation to spend pursuing their cognitive development. As well, parents who are not struggling to simply meet children's basic needs have the luxury of energy and time they can spend reading to children, playing games with them, and becoming involved in their homework and school related activities. This extra time and attention builds children's intellectual skills and also communicates to children that education is highly valued and important, giving them an advantage over children whose parents cannot provide such attention.
Educational Influences on Intelligence
Researchers have also found that the amount of time children spend in school is highly related to IQ scores. The more time children spend in school, the higher their IQ scores tend to be. The likely explanation for this finding is that teachers train children to answer factual questions, solve problems, and learn specific bodies of knowledge which then prepares them to answer questions appearing in IQ tests which have been formulated along similar lines and which draw upon the same bodies of knowledge taught in school. It follows that children who have more frequent exposure to educational environments are more prepared to respond to IQ style questions in a testing situation, and thus tend to do better on IQ tests than children who do not have as much of this sort of preparation. Therefore, children who miss out on educational opportunities because they're often absent or truant from school, or because they change schools frequently can end up disadvantaged when taking an IQ test.