The Divisions of the Nervous System

Humans, like other living organisms, have two control systems in order to respond to the environment:

  • The nervous system
  • The endocrine systemt

The organisation of the human nervous system:

  • The Central nervous system (CNS), which consists of the brain and spinal cord
  • The Peripheral nervous system (PNS) which consists of millions of neurons that carry messages to and from the CNS. These neurons are motor,sensory and interconnecting (relay) neurons.
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Motor Neuron

Motor neurons (or efferent neurons) carry messages AWAY from the brain and spine (CNS) to the organs and muscles in the body

A MOTOR neuron has a cell body with many dendrites branching off it. These have a large surface area in order to connect with other neurons and carry nerve impulses towards the cell body.

The axon then carries the nerve impulse away from the cell body. The legnth of axons vary. Surrounding the axon are special cells known as Schwann cells that wrap around the axon to form an insulating layer called a myelin sheath.

The axon divides into a number of branches at the end, known as synaptic terminals. These do not actually touch the next neuron, there is a small gap called the synapse.

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Sensory Neurons

Sensory neurons (afferent neurons) carry messaes FROM the receptors in the body (PNS) to the brain and spinal cord.

Receptors such as our sense organs, muscles, skin or joints detect physical and chemical changes in the body and relay these messages via sensory neurons to the brain or spinal cord

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Interconnecting Neurons

Interconnecting (or relay) neurons are found only in our visual system, brain and spinal cord.

These neurons recieve messages from the sensory neurons and pass these messages to other inconnecting neurons or to motor neurons. (

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The structural and functional differences

Motor Neuron - carries messages from the CNS to effectors such as muscles and glands. Has short dendrites and long axons

Interconnecting neuron - Transfers messages from sensory neurons to other interconnecting neurons, or motor neurons. Has short dendrites and short or long axons.

Sensory Neuron - Carries messages from the PNS to the brain and spinal cord. Has long dendrites and short axons

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Synaptic Transmission

Estimated that the human brain contains around one billion neurons and even more synapses. A synapse is a specialised gap that allows electrical messages from one neuron to transfer to an adjacent neuron. This is synaptic transmission.

When the nerve impulse travels down an axon, it arrives at a pre-synaptic terminal. This arrival triggers the release of neurotransmitters - chemicals that diffuse across the synapic cleft to the adjacent, post-synapic neuron.

When released, the neurotransmitter much be taken up immediatley or it will be reabsorbed by the synamptic terminals or broken down by enzymes. 

If sucessfully transmitted, the nerve impulse is then carried long the post-synaptic neuron until it reaches the next synamptic terminal where the message will continue to pass on via electrical impulses.

Neurotransmitters such as dopamine or serotonin cn influence the post-synaptic neuron to respond in an inhibitory way (decreases firing of a cell) or an excitatory way (increases the firing of a cell). Schizophrenia, for example, is a mental disorder thought to be the excessive activity of dopamine. They transfer too many messages throughout the brain-resulting in hallucinations. Chloropromazine is designed to block the recpetor sites for dopamine.

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Synaptic Transmission

Estimated that the human brain contains around one billion neurons and even more synapses. A synapse is a specialised gap that allows electrical messages from one neuron to transfer to an adjacent neuron. This is synaptic transmission.

When the nerve impulse travels down an axon, it arrives at a pre-synaptic terminal. This arrival triggers the release of neurotransmitters - chemicals that diffuse across the synapic cleft to the adjacent, post-synapic neuron.

When released, the neurotransmitter much be taken up immediatley or it will be reabsorbed by the synamptic terminals or broken down by enzymes. 

If sucessfully transmitted, the nerve impulse is then carried long the post-synaptic neuron until it reaches the next synamptic terminal where the message will continue to pass on via electrical impulses.

Neurotransmitters such as dopamine or serotonin cn influence the post-synaptic neuron to respond in an inhibitory way (decreases firing of a cell) or an excitatory way (increases the firing of a cell). Schizophrenia, for example, is a mental disorder thought to be the excessive activity of dopamine. They transfer too many messages throughout the brain-resulting in hallucinations. Chloropromazine is designed to block the recpetor sites for dopamine.

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Localisation of Cortical Function

The human brain is estimated to contain more than 100,000 kilometres of axons which recieve around 3 X 10/14 synapses. 

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Phineas Gage (1848)

A study was done to explain the cause of Phineas Gage's change in personality. 

Whilst working on a railroad, 25 year old Gage was preparing to blast a section of rock using explosives to create a new railway line. Gage accidentally dropped his tamping iron onto the rock which caused the explosive to ignite.

The explosion hurled the metre-legnth iron pole through Gage's left cheek, passed behind his left eye, and exited his skull and brain from the top of his head. The pole was found so metres away covered in bits of Gage's brain.

Miraculously, Gage survived and looked to regain his old railway job. However, no one would employ him as his personality had changed from someone who was kind and reserved, to one who was boisterous, rude and grossly blasphemous.

As Damasio et al state, although Gage's accident was horrific, it has taught us a great deal about the complexity of psychological processes that occur in the human brain.

We must be careful generalising, as was only based on one unfortunate individual. Additionally, the trauma of the accident may have caused this change in personality.

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Broca and Wernicke

During the 19th century, scientists such as Paul Broca and Karl Wernicke discovered that certain areas of the brain held particular functions - known as LOCALISATION of cortical function (specific areas of the cerebral cortex are associated with particular physical and psychological functions).

They also found that some functions, such as speech and language, where controlled by a particular hemisphere (side of the brain) - known as laterisation of cortical function (the dominance of one hemisphere of the brain for particular physical and psychological functions)

The human brain can be viewed as being formed of three concentric layers:

  • the central core, which regulates our most primitive and invoulantary behaviours
  • the limbic system which controls our emotions
  • the cerebrum which regulates our higher intellectual processes
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The Central Core

The central core is AKA the brain stem and controls our most primitive behaviours such as sleeping, breathing or sex, as well as invoulantary behaviours, such as sneezing.

The central core includes strucutures such as the hypothalamus. The hypothalamus is located in the midbrain and regulates our eating, drinking and sex, as well as regulating the endocrine system in order to maintain homeostatis.

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The Limbic System

Around the central core, and closely interconnected with the hypothalamus is the limbic system, which contains structures such as the hippcampus, which is thought to play a key role in memory.

This was discovered in the '50s when participants such as HM has their hippocampus surgically removed in order to treat severe forms of epilepsy. Upon recovery, HM suffered from severe form of anterograde amnesia.

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The cerebrum

The cerebrum has an outermost layer known as the cerebral cortex.

The cortex appears grey because of the location of cell bodies - which is why it is known as grey matter. Beneath the cortex lies myelinated axons which appear white - known as white matter. Each of our sensory systems sends messages to and from this cerebral cortex.

The cerebrum is composed of the right and left hemispheres which are connected by a bundle of fibres called the corpus callosum. This enables messages that enter the right hemispehere to be conveyed to the left hemiphere and vice versa. Each hemisphere is divided into four lobes:

  • the frontal love - the location for awareness of what we are doing within our environment (the consciousness)
  • The parietal lobe - location for sensory and motor movements
  • The temporal lobe - location for auditory ability and memory acquisition
  • The occipital lobe -location for vision
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The Motor Area

Located in the Parietal lobe and is responsbile for controlling our voulantary movements. Movements on the right side of the body are controlled by the left hemipshere and vice versa.

Therefore, damage to the motor cortex results in impaired movements

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The Somatosensory Area

Also located in the Parietal lobe, and seperated from the motor area by the central sulcus.

This responds to heat, cold, touch, pain and our sense of body movement. The amount of somatosensory area associated with a particular part of the body is related to its use and sensitivity. 

Eg, rats, which move around their environment using their highly sensitive whiskers, have a seperate cortical area for each whisker!! In humans, our face and hands compromise more than half of our sensory cortex.

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The Visual Area

At the back of the brain lies the occipitial love, whose primary function is VISION. 

Predominatley, nerve fibres from the inner half of the retina of each eye cross at the optic chiasm (the point at which the nerve fibres from both eyes converge) and travel to opposite sides of the brain.

As a result, damage to the left hemisphere can produce a loss of vision to the right side of our environment. Nerve fibres from the outer edge of each retina do not cross at the optic chiasm, and so damage to the left optic nerve can affect the left eye.

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The Auditory Area

Located in the temporal lobe and is responsible for the analysis of speech-based info.

Within this love, is an area known as Wernicke's area, named after Karl, who discovered that damage to the left temporal lobe resulted in linguistic deficts.

Individuals who experience difficulties in language comprehension suffer from what is known as Wernicke's aphasia.

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Split-Brain Research

Split- brain patients are patients that have undergone a corpus callosotomy, that is a large part of the corpus callosum that is lesioned. 

It was used in the '50s mainly to treat severe forms of epilepsy. As a result, the two hemipspheres of the brain are not able to communicate as effectivley. In the '60s Sperry et al conducted a vast amount of research on split-brain patients by testing various cognitive and perceptual processes.

They administered tasks known to be associated with each hemisphere. They discovered that the two halves of the brain were able to function quite independently. 

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Methods of studying cortical function

Psychologists have developed many methods of studying cortical specialisation in the brain in order to be able to predict, control and explain human and non-human behaviour.

One of the very earliest methods of studying the brain was proposed by Gall, who developed a technique called phernology - feeling for 'bumps' on the head. This was extremley popular in the 19th century, but was discredited for its lack of scientific evidence.

Today we use a variety of invasive and non-invasive methdos in order to explore and develop our understanding of areas of cortical specialisation.

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Post Mortem Studies

Post -mortems are a research method where the brain of a patient, who has usually been the subject of a longitudinal study because of some rare affliction, is examined after death.

The area of the brain that is damaged is then attributed to the affliction suffered by the person during their life. Post mortems have been used for centuries. Before the intro of contempory scaning methods, it was one of the few ways to study the relationship between the brain and behaviour.

Broca used post mortems to investigate the location of speech productionin the brain. One of his first patients were names 'Tan' at his unablility to speak any other word than 'tan.'  

Through an autopsy, Broca discovered that Tan had a lesion in the left cerebral hemiphere caused by syphilis. This area became known as Broca's area and damage to this section can lead to Broca's aphasia.

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EEG (Electroncephalogram)


EEG is a non-invasive measurement of electrical activity of the brain by recording from electrodes placed on the individual's scalp. 

Represents an electrical signal from a large number of neurons within the brain and the voltage differences between different parts of the brain is recorded. 

The filtered signal is then displayed on a computer screen, which is monitored.

Neuroscientists and psychiatrists use EEGs to study brain function by recording brain activity in humans and non-humans during various lab experiments.

EEG patterns recorded during sleep sessions, which are usually conducted in a specialised sleep lab, have contributed significatly to theories of sleep behaviour.

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(CAT) Computerised axial tomography


Where a narrow X ray beam is sent through the patient's head and the amount of radition absorbed is measured. These measurements can be made on hundreds of different axes through the head. 

These measurments are then fed into a computer, where a cross-section of the brain can be photographed or displayed on a screen. CAT scans are useful for evaluating the amount of swelling due to tissue damage, or assessment of the size of the ventricles (fluid-filled gaps) located deep within the brain.

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(PET) Positron emission tomography


A procedure whereby different levels of neural activity are assessed whilst the brain is active. A small amount of radioactive glucose is injected into the person's bloodstream and, after a few minutes, the brain begins to use the radioactive gulcose in the same way it uses glucose, providing the brain with energy.

The PET scan then detects and measures the amount of radioactivity emitted when individuals are asked to perform task such as solving problems.

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(MRI) Magnetic Resonance Imaging


Scanners use strong magnetic fields and radio waves to produce a high quality image of an individual's brain. During the procedure, the individual lies in a tunnel surrounded by a large magnet which produces a strong magnetic field. 

When a certain area of the body is exposed to a radio frequency pulse the tissues in the body give out a signal that is then measured. Like a CAT scan, hundreds of measurements can be made to produce precise images of the brain. MRI scans have been particularly useful in diagnosing diseases of the brain and spine.

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Evaluation of Methods

Post mortem - Provides a greater understanding of rare afflictions in individuals. But, obtaining a person's brain, even if they have been the subject of a  longitudinal study, is very difficult.

EEG - No intervention is necessary, and so allows for natural measurements of brain activity. However, electrodes are not sensitive enough to pick out individual action potentials of single neurons

SCANS - Provides deatiled knowledge of areas of the brain that are active whilst completing tasks such as problem solving. But, some scans are time consuming, so therefore unable to record spontaneous behaviour. There are also ethical issues surrounding the injection of radioactive glucose (PET SCAN).

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The Autonomic Nervous System

The perieral nervous system has two divisions:

  • The Somatic nervous system - controls skeletal muscles and recieves info to and from sensory receptors
  • The autonomic nervous system - maintains homeostatis by controlling glands and vital muscles such as the heart, stomach, blood vessels etc. It is 'autonomic' becuase the system operates involntarily

The two main divisions of the autonomic nervous system are:

  • The sympathetic nervous system
  • the parasympathetic nervous system

Their actions are mostly antagonistic; that is they usually work in opposition to each other, apart from during se, where the male's erection is due to parasympathetic action, followed by ***********, which is a sympathetic action.

The sympathetic nervous system functions when quick action is required (eg, during a threatening situation), whereas the parasympathetic system doesn't require immediate action. The sympathetic can be considrered the 'fight/flight' and the para the 'rest/digest' system.

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Actions of the Divisions

Sympathetic nervous system - 

  • Dilates pupils
  • Inhibits saliva production
  • Dialates bronchi
  • Speeds up heart rate 
  • Inhibits digestion
  • Stimulates glucose production
  • Inhibits urination

Parasypathetic nervous system -

  • Constrict pupils
  • Stimulates saliva production
  • Constricts bronchi 
  • Slow down heart rate
  • Stimulates digestion and bile
  • Stimulates urination
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Endocrine System

Generally, the sympathetic nervous system stimulates the fight or flight response to a threatening situation, whilst the parasympathetic nervous system resotres the body to its normal state.

The endocrine system is composed of a number of glands that release hormones directly to the bloodstream. Unless the fast acting nervous system, the endocrine acts more slowly to transort these hormones around the body.

One of the major endocrine glands is the pituitary gland, where in a stressful/threatening situation, it releases  an ACT hormone which is the body's major stress hormone  which stimulates the adrenal glands to release adrenaline directly into the bloodstream. 

In conjunction with the sympathetic nervous system, the adrenalin aids the fight or flight response by constricting blood vessels in the stomach, which inhibits digestion by increasing heart rate. 

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Effects of the stress Response

Prolonged exposure to these stress hormones can be damaging physically and psychologically.

Common side effects include disruption to one's sex life, problems associated with digestion, and in more severe cases, heart disease. 

Although our autonmoic nervous system is thought to be involuntary, this isn't necessarily the case when observing the practices of Zen Buddhists.

During meditation, they are able to control a number of autonomic functions, including heart rate and oxygen consumption. These physical changes exceed that which occurs during sleep or hypnosis. They can control any potential negative effects of stress.

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Genotype and Phenotype

Genotype = the genetic make-up of an individual. Refers to an individual's genetic make-up, that is the particular set of genes that the individual possesses.

Phenotype = the characteristics shown by the individual that are a resut of both genes and the environment. These are the traits shown by the individual, eg, height, weight and eye colour

The individual's genotype is the major influencing factor in the development of their phenotype, but it is not the only one. Phenotype is affected not only  by genes, but also the environment;

Genotype + Environment = phenotype

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If identical twins, who share the same genotype, were seperated at birth and raised in different environments, one twin may show a completley different phenotype from the other twin.

Eg, if one twin was fed a more nutritious diet, it would be physically much taller and stronger than the other twin.

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The difference

Psychologists have referred to various disorders in an attempt to explain the difference between genotype and phenotype. An example of such is the disorder of phenylketonuria (PKU) which is characterised by a deficiency in the enzyme phenlalanine hydroxylase (PAH). 

PKU is a recessive genetic disorder, which means each parent must have at least one defective gene for PAH, which the child then inherits. It is possible for a parent with PKU phenotype to have a child without PKU if the other parent has a functional PAH gene. A child who has two paretns with PKU will always inherit two defective genes and therefore the disorder.

If undetected and untreated at birth, individuals tend to fail to accomplish important developmental milestones. But, if PKU is diagnosed early, a newborn can develop normally if given a special diet, low in phenylalanie.

Also, haemophilia is a recessive, genetic illness that impairs the body's ability to control blood coagulation. A heterozygous (the genotype consists of two different genes Bb) indivdual is a carrier. The individual has a normal phenotype, but has a 50:50 risk of passing the gene onto its offspring. A homozygous (genotype is two genes that are the same, eg BB) dominant individual has a normal phenotype, and no risk of passing the gene onto offspring. A homozygous recessive person has an abnormal phenotype and guarranteed to pass the gene to offspring.

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Monozygotic Twins

Monozygotic (or MZ) twins are identical twins.

These occur hwne a single egg, which is fertilised to form a zygote, divides into two seperate embryos. These two embryos continue to develop into doetuses whilst sharing their mother's womb. If the zygote divides into two embroys at around two days, the embryos may develop seperate placentas and aminiotic sacs. 

Usually, the zygote divides after two days, resulting in a shared placenta, but two seperate amniotic sacs. Around 1 in 50,000 pregnancies result in conjoined twins - thought to be due to the zygote dividing too late. Around 13 days after feritlisation.

MZ twins are genetically identical, and share exactly the same DNA. Although they generally look similar, MZ often become less alike as they grow older due to lifestyle choices etc.

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Dizygotic Twins

Dizygotic (or DZ) twins are non-identical, fraternal twins.

These occurs when two eggs are cells are fertilised by two different sperm cells. Unlike MZ twins, DZ twins are no more genetically alike than are ordinary siblings (brothers and sisters).

Psychologists are interested in studying monozygotic and dizygotic twins to investigate the genetic basis of behaviour.

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Investigating the Genetic Basis of Behaviour

The field of behaviour genetics uses both genetics and psycholoy to study whether behavioural characteristics are inherited.

It is evident that physical characteristics, such as eye colour, height, etc, are inherited from our parents. 

Psychologists are interested in whether characteristics such as intelligence, sexual orientation, additiction etc are also inherited.

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Twin Studies

Certain traits run in families, eg, musical ability.

However, the problem in assuming that this ability must be heritable is that familes also share the same environment. We are hence left unsure as to whether musical ability is a result of inheritence or parental influence. Psychologists have looked at twin studies to try and answer this.

The rationale is that if one MZ twin has a particular characteristic, and so does the other twin, then that characteristic may be genetic as they share exactly the same genotype: there should be 100% concordanace (agreement between; the extent to which a pair of twins share similar traits or characteristics). 

DZ twins, who don't share the same characteristics, should show a much lower concordance rate. If the environment plays a significant role in determining our behaviour, then MZ twins, who have been reared apart, should show a low concordance rate for traits such as intelligence.

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Intelligence = Wilson conducted a longitudinal study and found that, at 18 months, MZ twins were more similar than DZ twins on tests of intelligence.

The follow-up data showed MZ twins were more similar in their intellectual performance. Plomin tried to account for these differences by stating that their different genotypes may have directed them along seperate developmental paths, accounting for their 'poorer' intellectual performance, compared to MZ twins who share the same genotype.

Sexual orientation = Some evidence from twin studies sugest homosexuality may be genetically determined. Bailey and Pillard, in a study of male twins where at least one was gay, found that 52% of MZ brothers were concordant for homosexuality compared to 22% of DZ brothers. Baiey, Dunne and Martin found a 30% concordance for homosexuality in MZ twins. 

Criticisms have included the way in which these participants were recruited (eg, through gay media, whose target audience is clearly homosexual.) This would account for the high response rates to take part in suh studies compared to the potential recruitment of homosexuals through traditional media. Bearman and Bruckner argue against genetics, and argue such low concordance rates don't account for genetic similarity (eg, 30%/30%/0%/7.7%) Overall, most suggest there isn't a single 'gay gene.' Our sexual orientation is likely to be a result of a combination of genetic and cultural factors.

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Family Studies

Francis Galton in 1869, assumed all natural abilities were inherited. By his own admission, Galton had to conclude that any resemblance between family relatives could be a result of both genes and environment.

Addiction - Krishnan et al (1996), found that sons of alcoholic fathers were more likely to be alcoholics themselves, when compared to people selected at random. The researchers discovered when sons of alchoholics drank alcohol, they tended to release more of the neurotransmitter endorphin compared to others, suggesting a biological preposition to alcholism.

Family size + Birth Order - Evidence that the environment plays a more significant role than genes in determining behaviour. Eg, Zajonc and Markus resaerched IQ data of 40,000 Dutch males born in 1944. They found IQ is related to birth order and family size.

The researchers suggested that this was largely due to the degree of attention given by parents: larger families may mean that each child has a smaller amount of parental attention and perhaps more of a physically deprived environment; hence, a lower IQ.

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Adoption Studies

Adopted children in one family may be compared with biological children in another family, or studying families with both adopted and biological children. If a trait or characteristic has a genetic basis, then the adopted child should show the same trait as its biological parent. However, if a trait or characteristic is environmental, then the adopted child should show similar characteristics to their adoptive parents.

  • Age = As children grow older, it is assumed their cognitive and verbal abilities would develop to become more like their adoptive parents, than their biological parents. However, Plomin et al found, as adopted children approached 16, they became more similar to their biological parents in cognitive and verbal ability compared to their adoptive parents, hence suggesting a genetic influence.
  • Intelligence = Scarr et al found evidence that intelligence was strongly linked to environmental influence. Black low class children were adopted into white middle class families where the adoptive parents had at least one biological child. They initially found the black children were more intellectually similar to their biological parents (0.43) than their adoptive parents (0.29), supporting a genetic basis. However, they found inter-racial siblins also showed intellectual similarities. As these inter-racial siblings have no genes in common, this must be due to the environment.

Adoption studies must be viewed with caution - they are usually smaller, richer and may provide a more stimulating environment compared to some bio parents. Account for higher IQ?

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Selective Breeding

One method of studying genetics is that of selective breeding.

This involves artificially selecting male and female animals for a particular triat. These animals are then put together in order to breed and produce offspringg. The advantage is that it is a quick way to select for particular traits, wih the effects of artificial selection being seen within just a few generations of breeding. Plomin suggests if selective breeding doesn't alter the trait or characteristic, then we must assume that this is entirely dependent on environemtal factors.

Tryon conducted an experiment with the aim of investigating whether genetics influenced learning in rats. He trained a large number to run a complex maze. Those that were quickest were selected as well as the slowest. He then bred the 'bright' rats with other 'bright' rats and 'dull' rats with other 'dull' rats. He continued this breeding process for a number of generations.He measured the number of errors made and speed which the rats learned through the maze.

'Maze bright' rats learned to run the maze faster and made fewer errors compared to 'maze dull' rats. This enforces that learning is heritable which can be controlled through selective breeding. We must be careful in generalising though as humans act very differently in their environment.

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Evaluation selective breeding

Can't generalise

Cooper and Zubeck conducted a similar study but found when rearing 'maze dull' rats in:

  • An impoverished/boring environment with a barren wire mesh cage
  • A stimulating environment containing tunnels, ramps etc

When rats reached maturity, the 'maze dull' rats that had been reared in a stimulating environment made the same number of learning errors as the 'maze bright' rats in a stimulating environment. This displays that the environment is also an important factor in determining behaviour.

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A bit extensive, but very informative and well structured. Thanks for the resource.






I accidently rated this a 1 but was meant to rate it a 5. This had really helped me. Thank you **



estimated 100 billion neurons in the brain 

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