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Seattle Longitudinal Study



Background
Seattle Longitudinal Study is one of the most comprehensive researches on how humanity’s intelligence and ability change throughout their life span [1]. This study, conducted by Dr. K. Warner Schaie and his colleagues, is based on Thurstone's theory of psychometrics. Inspired by his mentor Professor Charles R. Strother, Dr. K. Warner Schaie emphasized individual differences and the effect of antecedent variables on personal abilities [2]. This study described the correlation between changes in adult cognitive ability and age, and obtained data on the magnitude of the change. In addition, it also confirmed the relationship between the individual's living environment conditions, health status and personality characteristics and the development of cognitive ability, thus providing a conceptual basis for possible measures to delay cognitive decline [3].

The Key Researchers
Dr. K. Warner Schaie is the founder of Seattle Longitudinal Study. He is an American psychologist with strong influence in the fields of psychology and gerontology. After obtaining his bachelor's degree in City College of San Francisco, he started a journey in the field of social science. Later, Schaie entered into the University of California at Berkley and University of Washington at Seattle specializing in psychology. During the period of doctoral study, inspired by his mentor Professor Charles R. Strother, Schaie conducted this long-term research based on Thurstone's theory of psychometrics. For his great contribution to psychology science, the American Psychology Association gave him the Distinguished Scientific Contributions Award [4]. Dr. Sherry Willis, a life-span developmental psychologist concentrating on age-related cognitive development, joined the Seattle Longitudinal Study in 1984. In 2004, she became the co-director of this research [5].

Research objectives
Seattle Longitudinal Study attempts to answer 5 issues [1]:

1.	Does intelligence influence uniformly throughout adulthood, or are there diverse life-course-ability patterns?

2.	Can decrement in ability be reliably detected at any magnitude at any age?

3.	What are the generational differences for patterns and dimensions?

4.	How to explain differences in age-related change in adulthood for individuals?

5.	Can the decline in mental with age increasing be counter by educational intervention?

Methodology
1.	Longitudinal Study

Dr. K. Warner Schaie and his colleagues spent almost 50 years to investigate the changing process of intellectual abilities during adulthood and the causes of changes. Starting with his doctoral thesis in Washington University in 1956, Schaie had completed seven tests so far, respectively in 1956, 1963, 1970, 1977, 1984, 1991 and 1998. Each test took approximately 7 years [3].

2.	Data Collection

From 1956 to 1998, Schaie and his colleagues tracked more than 6000 adults aging between 22 and 101 and assessed their cognitive abilities throughout the study. The participants lived in Seattle, Washington and other large cities. The majority of them were members of Health Maintenance Organisation and the Group Health Cooperative of Puget Sound as well as the family members in their households. Even though the sample was not sufficient to represent the population with local socioeconomic status, it covered almost 75% of population in the society [3].

3.	The Measurement Variables

Schaie concentrated his research on the five primary cognitive abilities according to Thurstone’s SRA Primary Mental Abilities Test [6]. During the study, Schaie and his colleagues designed or used a number of measurements focusing on the five primary cognitive abilities:

erbal Meaning refers to a person’s ability to understand the meaning of ideas expressed by the words. It reflects the vocabulary range that has been activated [6]. The measurements used to test verbal meaning include ETS Advanced Vocabulary [7], PMA Verbal Meaning [6] and ETS Vocabulary [7].

Inductive Reasoning refers to a person’s ability to identify and understand new concepts as well as the relations between them. It allows a person to predict and plan how to solve logical problem [6]. The measurements used to test inductive reasoning include Word Series [8], PMA Reasoning [6], Number Series [7], and ADEPT Letter Series [9].

Spatial Orientation refers to a person’s ability to visually and psychologically control the structure of three- or two-dimensional space. The measurements used to test spatial orientation include Cube Comparisons [7], PMA Space [6], Alphanumeric Rotation [1] and Object Rotation [8].

Number refers to a person’s ability to understand the relations between numbers, process data, and deal with simple quantity problems in a fast and accurate manner. The measurements used to test number ability include Subtraction & Multiplication [7], PMA Number [6] and Addition [7].

Word Fluency refers to a person’s ability to recall languages and vocabularies during writing and conversation. The measurements used to test word fluency include PMA Word Fluency [6], Immediate Recall [10] and Delayed Recall [10].

Findings
1.	The Different Changing Patterns of Intelligence During Adulthood

Schaie and his assistants have clearly demonstrated that the pattern of age-related changes differ in all intellectual abilities during adulthood. They also noted that more research need to be conducted in order to develop a better understanding of this finding. Thus, in 1991, the researchers conducted cross-sectional data collection and analysis focusing on five basic cognitive abilities, and found that: (a) Among these abilities, the peak of inductive reasoning, spatial orientation, and word fluency appears in the early adulthood (about 25 years old), while the spatial orientation and inductive reasoning ability decreased with age, and the curves of the two declined most. The decline in the word fluency curve was not as significant as the former two, but overall it was linear. (b) The peak of verbal meaning and number appears in middle age (about 39 and 45 years old, respectively). However, in some tests that require quick answers, verbal meaning shows negative age differences before the early years of age [3]. Besides, Schaie also revealed the gender difference trends and pointed out that compared with men, women’s fluid abilities may decline earlier while crystallized abilities may decline later [3].



2.	The Age and Magnitude of Detectable Age Decrement in Ability

Schaie and his assistants, based on sufficient data, found that people’s cognitive abilities, except for verbal meaning, begin to decline significantly around 53, 7 years earlier than previous findings. They also demonstrated that before 67, all the cognitive abilities decline but in a slow manner until 80. However, after 80, fewer than half of participants showed significant cognitive decline in the following 7 years. The researchers attributed the cognitive decline in later life to the reduction of processing and reaction speed. This finding contributes to social development by providing an important theoretical basis for policy making on a series of social issues, such as compulsory retirement schemes and age discrimination in employment [3].

3.	The Patterns and Magnitude of Generational Differences

Through the tests of single intellectual ability among participates of the same age but born in different years, Schaie and his assistants demonstrated that there is significant general differences on cognition abilities [3] [11]. They observed that inductive reasoning and verbal meaning are positively linear related intergeneration, while the positive linear relation between spatial orientation and intergeneration is discontinuous. Additionally, they found that the peak of number ability shows among participants who were born in 1924 and a declining trend was found among participants born later. Compared with participants born earlier, the latest generation tends to have lower word fluency as well. Based on these findings, Schaie concluded that previous transverse research either overrated or underrated the age-related cognitive decline of adults before their 60s [3].

Regarding the increase of certain cognition ability along with generational development, Schaie attributed this change to the improvement of formal education level. He noted that the development of teaching strategy and use of teaching materials may be important factors influencing the generational difference [3].

4.''Antecedent Variables Leading to Individual Differences in Age-Related Change in Adulthood '' Schaie and his assistants identified some antecedent variables leading to individual differences in age-related change during adulthood, including being free from cardiovascular disease and other chronic diseases [3][12], living in comfortable environment supported by higher socioeconomic status [3][13], engaging in activities that require a great amount of complex intellectual participation [3], maintaining healthy personality [3], marrying a person with high cognitive level [3], and doing self-evaluation according to their satisfaction with life achievement in early life [3].

5.	The Effectiveness of Educational Intervention on Reversing and Delaying Intellectual Decline

Through the long-term tracking, Schaie and his assistants were able to design some intervening measures to help delay and even reverse intellectual decline. They provided trainings on the five basic cognitive abilities for participants and tested their cognitive level before and afterwards. Participants were found significant improvement on cognitive abilities after the intervening measures. For those participants who showed cognitive decline before implementing the intervening measures, the effects of training are more obvious. Schaie also found that male participants got more benefits from trainings on inductive reasoning while female participants benefited more from spatial orientation [14].

6.	The Relation between Cognition, Health and Mortality

Although all the intervening measures were implemented in laboratory setting, they are highly relevant to the cognitive abilities required in everyday life. Schaie concluded that the cognitive decline among the elderly in many residential areas may be a result of neglecting intellectual activities. In other words, the age-related cognitive decline is reversible [3].

7.	Changes in Aging Rate

Schaie and his assistances also collected 7-year longitudinal data from parents and their adult children based on the assessment of cognitive ability at the same ages. They found that participants’ cognitive ability of verbal meaning, inductive reasoning, spatial orientation, and reaction speed decline less significantly during their early old age. This finding supplemented the previous study on intergenerational difference [1] [3].

8.	Early Detection of Risk for Dementia

Schaie and his assistances collected data from more than 500 participants in their 60s or 70s. They found that the majority of participants who later suffered dementia could have been predicted based on the data collected seven or fourteen years earlier [1] [3].

Recent Development
The Seattle Longitudinal Study is still being continued today. The successor study is named "Midlife Cognitive Change and Risk of Cognitive Decline". In 2005, another test was carried out among over 1200 participants who were assessed in the test in 1998. From 2011 to 2013, the researchers conducted the tenth cognitive test in this study among participants from the newer generation [15].