Nature and nurture play a significant role in determining the life of people with regard to their intelligence level. Intelligence is the unique way in which a person can be able to come into an understanding of problems mentally, respond to them and replicate the surrounding he exists. Human intelligence is determined by two main factors which have caused great debate for years. These two factors are the environmental also known as the external factors and the genetic factors also referred to as the internal factors. Scientists have been arguing for centuries about which particular factor influences the human intelligence. Some scientists believe that both the environmental and genetic factors influence the human intelligence, but are currently having a debate on which factors has significant impact. This paper provides a detailed analysis of the relationship between intelligence and the internal and external factors influencing it. Genetic factors are hereditary and are transferred to offspring through their parents. Various experiments have been carried out to determine whether if it is true about the internal factors having an influence on the intelligence of human beings. The outcomes of these studies have been positive, showing that genetics have some influence on the level of intelligence (Hughes and Cutting, 1999). The major experiment that proved existence of this relationship is the twin experiment. In this experiment they picked two pairs of identical twins, placed the first pair in a similar environment, and then separated the other pair of twins placing them in different environment. After some years a test was done on their level of IQ. Those identical twins that were placed in the same environment showed almost similar IQs, but the pair of twins who were separated had different IQs with a wider margin. Nonetheless, this proved the influence of genetics on human intelligence. There have been other studies which involved the fraternal twins and the identical twin placed under the same environment. It was observed that the level of intelligence in the identical twins was almost similar, but in the fraternal twins, the intelligence level had a big difference (Singh, 1996). Another study which has also been a major indicator of the influence of genetics to intelligence has been the use of identical twins and an adopted child. The twins were separated to live in two different environments, but one of the twins was to live in the same environment as the adopted child. After a certain span of time their IQ level are matched. The results obtained showed that although the intelligence of the twins varied, it would be with a small margin compared to the margin that would be seen by the two unrelated kids living under the same environment. The study indicated that the internal factors accounted for approximately 25% of human intelligence while the remaining 75% was influenced by the external factors (Hughes and Cutting, 1999). The environmental factors that influence the intelligence possessed by individuals refer to the sum total of all the circumstances that surrounds a person. These include the society one lives in, the culture that is usually practiced by the people around, and also the life experiences that an individual goes through from childhood to adult hood. Some of the environmental factors affect individuals while the others affect the population at large (Singh, 1996). Good examples of these factors are the culture and the social background in which an individual lives in. These two have a very big impact on the level of intelligence of human beings. The variation in families usually portrays itself in the background different families occupy. The difference in backgrounds usually affects the child’s mental growth in one way or another (Hughes and cutting, 1999). A very good illustration is when a child is brought up in a family that is very strict on matters pertaining to education. The child grows to treasure education and will always be working towards the increase of his intelligence. In many occasions, this effort bares fruits as the parents are always there to back up their child. Surveys carried out by research personnel have shown that kids who grew up in families that treasured education had a very high IQ. This was compared to families who placed much interest on their child obtaining primary skills as well as the survival skills. Children from these families were observed to have a very low IQ as they had no motivation to push them nor did they have the encouragement from their parents (Hughes and Cutting, 1999). The reason for this could be summarized by saying that the social background of an individual plays a significant part on the level of his intelligence. The environment usually has a lot of aspects that may determine the level of intelligence of an individual although there are main factors. The social aspects as mentioned above play a crucial role in the development of a child’s intelligence. When talking about the social aspect we are often very general as social aspect is a very big field. The social factors are mainly schooling, occupation, the family environment that one is living in and lastly the interventions made by parents. School is very crucial to the intelligence of a person (Singh, 1996). The level of intelligence can at times be affected by the number of years an individual has been in school. The number of years can be brought about by slow learning due to an individual’s low level of understanding, but the long schooling discussed here is that which comes with the urge to learn more. The more a person learns, the more the knowledge he gains. This acts as a vehicle in the improvement of the intelligence of a person. The occupation of an individual also determines his level of intelligence. Some occupations are concerned with research, while others are related to activities that involve the use of critical thinking or a creative mind. These types of occupation usually lead to the increase in intelligence level of individuals. These occupations compared to other jobs such as newspaper vending which involves less of the brain use. The family environment in which a child lives in may have some impact on the degree of intelligence that a child will develop to have (Singh, 1996). Parents and other members of the family may influence a child’s intelligence in different ways, one of them being the way they behave. In a situation where family members exhibit arrogance, and don’t take interest at all on their child, the child would up in isolation which affects his intellectual growth. Parents should take the responsibility of guiding their children in most of the things they do in their day to day life. Parental intervention plays a key role in the growth of the child’s intellect. A child is always in need of parental guidance as it always keeps him with the knowledge of what is right and what is wrong. Biological aspects affecting human intelligence are usually components that in a way alter the operation of the body and the outcomes, impact the level of intellect a person may have. The main biological variables are nutrition, diseases, lead, alcohol, and perinatal factors. The issue of nutrition is mainly focused on the diet given to a child. Usually childhood is the foundation of every individual and so the initial development of the brains at a young age is very important. If a child is malnutrition, he or she is affected mentally in that there is minimal mental growth and as a result they live to have a lower IQ level (Hughes and Cutting, 1999). Diseases mainly affect the brain in a negative way. Some diseases may affect the child brain and cause some distortion and as a result the child acquires some problems such as slow thinking or even becomes a slow learner. Therefore his or her intellectual level is brought down. A child may also encounter intellectual problems due to the perinatal factors. Perinatal is the period between 22nd week in gestation and 28 days after the birth of a child. There are complications that might arise if this period is not handled with utmost care (Singh, 1996). One of the issues arising from perinatal period is the issue of breastfeeding after childbirth. If two children born at the same time are used as specimen, with one of the child being exposed to breastfeeding, while the other is left without breastfeeding for a period of about one month, it would be observed that the intellect of the breastfed child would have increased with a greater range than the intellect of the child who was left without breastfeeding. In the recent past there have been very many debates concerning the effects of lead element to human health. Lead as one of the main pollutants by vehicle due to the leaded petroleum, has brought a lot of problems to the human beings. Lead affects the brain in that it causes the slowdown of brain functioning. This means that the intellectual level of an individual may be lowered. Alcohol consumption is also known to have the same effect to the brain. The drinking of alcohol alters the way the brain works and the consistence use causes the intellectual level of the victim to reduce (Singh, 1996). Test scores have been carried out and what has been observed is that the level of test scores has been on the rise recently. Flynn in his study of human intelligence concluded that the level of intelligence has been on the rise. He even goes further to explain his point by saying that if one of our great grandfather, who was the brightest during his time is brought to life, he would be one of the people with very low IQ (Singh, 1996). Nonetheless, Flynn also attributed this rise in IQ from the change in education system, and also from the complexity of life among other factors. In conclusion, the intelligence level is usually affected by the internal and external factors in a positive or negative manner. In the positive, these factors act to improve the intelligence of an individual whereas on the negative act to lower the level of intelligence in a person. Nonetheless, the part played by genes in determining intelligence is very significant than the environment. However, the effect of the environment cannot be ignored in that a child raised up in a family valuing education would indeed value education and vice versa.