Psychologists primarily use three types of research designs to study development, they are: cross-sectional, longitudinal and sequential.
Cross-Sectional Studies – Cross-sectional studies compare groups of participants of different ages at a single time to see whether differences exist among them. For example, a research group studied people who had reached 100 years of age and compared trhem on a number of dimensions with people in their 60s and 80s (Poon et al., 1992). Cross-sectional studies are useful for providing a snap shot of age differences, or variations among people of different ages. The major limitation of cross-sectional studies is that they do not directly assess age changes; that is, changes within the same individuals that occur with age. As a result, they are vulnerable to confounding variables. Cross-sectional studies are therefore most useful when cohort effects – differences among age groups associated with differences in cilture – are minimal.
Longitudinal Studies – Longitudinal studies assess the same individuals over time, providing the opportunity to assess age changes rather than age differences. The advantage of longitudinal studies is their ability to reveal differences among individuals as well as changes within individuals over time. Like cross-sectional designs, longitudinal designs are vulnerable to cohort effects. Because they investigate only one cohort, they cannot rule out the possibility that people born at a different time might show different development paths.
Sequential Studies – Sequential studies minimise cohort effects by studying cohorts longitudinally. In an ideal sequential design, researchers study a group of people at one age and follow them up over time. As the study progresses, a new and younger cohort is added to the study, beginning at the same age at which the first cohort began. Essentially, a sequential design combines cross-sectional and longitudinal comparisons, allowing researchers to distinguish between age effects and cohort effects. The only catch is that sequential designs take years or even decades to complete.
PHYSICAL DEVELOPMENT AND ITS PSYCHOLOGICAL CONSEQUENCES
Having examined some of the basic issues and methods of developmental psychology, we now turn our attention towards physical development and its impact on psychological functioning. Some of the most dramatic aspects of physical development take place before we are even born. Let’s have a look shall we?
One of the most remarkable aspects of human development is that a single cell, forged by the union of a sperm and an egg, contains the blueprint for an organism that will emerge – complete with billions of specialised cells – nine months later. The prenatal period is divided into three stages.
- During the germinal period – roughly 2 weeks post conception – the fertilised egg becomes implanted in the uterus.
- The embryonic period – 3rd – 8th week of gestation – is the most important period in the development of the central nervous system and of the organs. By the end of this stage, the features of the embryo become recognisably human, the rudiments of most organs have formed and the heart starts to beat it’s life long rhythm.
- During the foetal period – 9 weeks to birth – muscular development is rapid. By about 28 weeks, the foetus is capable of sustaining life on its own.
At birth, an infant possesses many adaptive reflexes. For example, the rooting reflex helps ensure that the infant will get nourishment: when touched on the cheek, an infant will turn her head and open her mouth, ready to collect milk. The sucking reflex is similarly adaptive: infants suck rhythmically in response to stimulation 3 or 4 centimetres inside their mouths . Many early reflexes disappear within the first six or seven months, as infants gain more control over their movements. In general, motor skills progress from head to toe: infants first master movements of the head, then the trunk and arms and finally the legs. Motor development in infancy follows a universal sequence, from smiling, turning the head and rolling over, to crawling, walking with support and finally standing alone and walking without help from any one.
Childhood and Adolescence
Some of the most important maturational changes that influence psychological development involves changes in the size and shape of the body. A remarkable aspect of human development is the extent to which children can maintain the sense that they are the same person over time despite massive changes in the way they look. Growth rates for girls and boys are roughly equal until around age 10. At that point, girls begin a growth spurt that usually peaks at age 12, and boys typically follow two or three years later. Individuals of both sexes vary, however, in the age at which they enter puberty, the time at which they become capable of reproduction. Girls usually experience the onset of menstruation at about age 11 to 13, while for boys, mature sperm production is somewhat later, about 14 to 15 years.
Adulthood and Ageing
By the end of adolescence, physical growth is virtually complete, and the changes that occur thereafter tend to be gradual and less dramatic. People often gain a few centimetres in height and several more centimetres in fat between ages 18 and 28. By their 30s, people are already deteriorating physically, with muscular strength and sensory abilities showing subtle but clear signs of decline. Individuals also differ tremendously, however, in the extent and pace of these changes, as some 80 year olds run marathons in their free time.
As in childhood, some of the most apparent signs of ageing are in physical appearance, such as wrinkled skin and grey hair. Sensory changes are also substantial, older adults have reduced sensitivity to visual contrasts – for example, climbing stairs can be difficult because they have trouble seeing where one step ends and another begins (Fozard, 1990). Older adults also take a longer time adapting to the dark, which can cause problems driving at night, as oncoming headlights may create temporary flashes of brightness. Hearing loss is also common. Many older people experience presbycusis, the inability to hear high frequency sounds, which can make hearing the phone ringing quite difficult. The inability to understand what others are saying can have disturbing psychological consequences. We often lose patience with old people who constantly ask us to repeat what we have said to them. Younger people may also inadvertently treat older people with hearing loss condescendingly, speaking slowly or more loudly. Deterioration in certain areas of functioning is an inevitable part of ageing, but development throughout the life span is characterised by gains as well as losses. Many Western images of the elderly stem from negative cultural myths and stereotypes, such as the idea that sexuality ends in the 40s or 50s and that old people are destined to become senile. These stereotypes are examples of ageism, or prejudices against old people. We all have to become old sooner or later, however, people generally don’t want to accept this ugly truth and so we instead outcast old people and treat them as though they are different from us.