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Cluster Analysis of Developmental Pathways of Divergent Reproductive Strategies Triin Anton & Bruce J. Ellis University of Arizona Background

Results

This research is theoretically grounded in life history theory. Life history theory (LH) takes the viewpoint that the behavior and physiology of organisms can be understood as the effects of natural selection on key maturational and reproductive characteristics or traits such as characteristics that determine rates of reproduction and patterns of growth, aging, mating behavior, and parental investment. While sexual development generally follows a speciestypical process in humans characterized by pubertal maturation, increases in sexual motivation and the onset of sexual activity, there exists substantial variation in the timing of these events between individuals. From an evolutionary perspective, this variation in timing is critical in regulating the development of alternative reproductive strategies. Optimal strategies vary across individuals within and between populations (see Ellis et al., 2009 for a review) and specifically, timing in sexual development and behavior can be characterized as being on a developmental continuum.

Examination of a dendogram indicated it was appropriate to interpret a two cluster solution. Participants were split 583 (49%) into cluster 1 and 616 (51%) into cluster 2 with 367 (48%) of boys and 216 (49%) of girls classified into cluster 1. Independent samples t-tests were performed to examine cluster membership and means on key variables (see Table 1). Consistent with the BSD model, it was found that participants in cluster 1 had significantly lower SES, lower relationship adjustment scale scores, higher average number of life changes, as well as significantly lower mean scores on parental consistency, parental warmth, appropriate parenting and significantly higher mean scores on parental harshness. Further, as an indicator of criterion validity , children who were identified as “high risk” by the Fast Track Project (both control and intervention groups) were significantly over represented in cluster 1 (51.2% versus 43.2%) and conversely, children identified as “normative” were over represented in cluster 2 (48.8% versus 56.8%) Χ2(1,1199)=6.8, p<.05. Based on these results, consistent with the BSD model, it was determined to name cluster 1 as Type I and cluster 2 as Type II.

Table 1: Mean Comparison on Key Variables Socioeconomic Status

In theory, for human children, family conditions and childrearing should serve as cues regarding the harshness and unpredictability of the environment that they are likely to mature into and children should, therefore, adapt their behavior and development accordingly. This assertion has been more explicitly developed by Belsky and colleagues in the Developmental Pathways of Divergent Reproductive Strategies model (BSD model; Belsky, Steinberg & Draper, 1991). The BSD model makes specific predictions about environmental characteristics in early development (first 5 to 7 years of life) and subsequent maturation and behavior.

Relationship Adjustment Life Changes Parental Consistency

The BSD model predicts two “Types” of reproductive pathways that roughly correspond to fast and slow life history strategies. Based on either harsh/unpredictable or sensitive/stable family context and parenting that is either harsh/insensitive or warm/supportive children are hypothesized to differ in their psychological and somatic development and eventually their reproductive strategy. Derived from the BSD model we expected to identify two clusters of children based on aspects of family context and childrearing. One cluster would be characterized by children whose family context and childrearing was harsh and unpredictable and the other of children who had more harmonious and sensitive family contexts and parenting. The children in these clusters should differ on subsequent somatic development and indicators of reproductive strategy.

Parental Harshness Parental Warmth Appropriate Parenting

Cluster

Mean

Std. Deviation

t

P-value

1

20.4

9.7

15.8

<.01

2

30.2

11.6

1

89.4

14.2

32.0

<.01

2

112.9

11.1

1

1.5

.9

4.3

<.01

2

1.3

.8

1

2.5

.8

14.8

<.01

2

3.1

.7

1

1.0

.4

9.9

<.01

2

.8

.3

1

2.4

.4

9.0

<.01

2

2.6

.4

1

2.5

.5

11.5

<.01

2

2.8

.5

N for Cluster 1= 583, N for Cluster 2 = 616, df for all t-test = 1197

Method Participants: The sample for this analysis was selected from the Fast Track project dataset (N=1199). Fast Track is multi-site intervention designed to prevent serious and chronic antisocial behavior in a sample of children selected as high-risk at school entry because of their conduct problems in kindergarten and home (fasttrackproject.org). The sample has data from 440 girls and 759

boys, all of whom were included in the analysis. Approximately 48% were White, 49% were African American, and 3% were classified as other (Hispanic, Asian, Native American).

Variables of Interest: Data for the ecological/family conditions in the current analysis were collected at year 1, 2, 3 of the study when the children were approximately 5, 6 and 7 years of age. Family Context was evaluated using: SES (based on Hollingshead formula, taking into account occupation and education) averaged across the first 3 data collection points and an averaged life changes score (composed of the average number of moves, job and parental changes until age 7) and the Relationship Adjustment Scale which assesses satisfaction in the relationship of spouses or domestic partners (28 items, averaged across first 3 time points). Childrearing was represented with the Parenting Questionnaire (27 items with subscales of: parental consistency, warmth, harshness, appropriate parenting, averaged across first 3 time points). At years 7 and 8 in the study (approximately 12 years of age for girls and 13 years of age for boys) adolescents and their parents each completed a questionnaire that combined the Pubertal Developmental Scale (PDS; Petersen, Crockett, Richards, & Boxer, 1988) with additional items from Morris and Udry’s (1980) Index of Adolescent Development (IAD). Data on pregnancies and sexual behavior was collected from children starting at 13 years of age until age 19.

Analysis: In order to maximize power and minimize exclusion of participants due to missing data, we used multiple imputation in Amelia II to impute missing values. All numeric variables were entered into the expectation maximization (EM) algorithm for imputation; measures of demographic characteristics (gender and race/ethnicity) were excluded from the imputation procedures (missingness ranged from 0 to 31%). A hierarchical agglomerative clustering technique was used with Ward’s method for linkage between clusters. Ward's method is a minimum distance hierarchical method which calculates the sum of squared Euclidean distances from each case in a cluster to the mean of all variables; the cluster to be merged is the one which will increase the sum of squared distances the least. Squared Euclidian distance uses the length of the hypotenuse of a right triangle formed between points (subsequently squared) to index similarity/dissimilarity between individuals. This particular technique was selected due to the inadequacy of correlational methods for accounting for mean differences, specifically since mean differences on continuous variables were thought to be of interest in this case. Ward’s method was selected because it is not prejudiced by correlations between variables and, according to the literature, the variables of interest in this analysis are generally highly correlated (e.g. SES and life changes). This method of clustering maximizes between group differences and minimizes within-group distances (see Aldenderfer & Blashfield, 1984 for an overview of cluster analysis).

To examine indicators of reproductive strategy, independent sample t-tests were conducted. Type I and Type II children’s mean stage of pubertal development at age 12 for girls and age 13 for boys (as well as age at menarche for girls) did not significantly differ. However, Type I participants, on average, had sexual intercourse earlier, had more teenage pregnancies and children by age 19. Type I males (but not females) were also more likely to report having contracted a STD by age 19 than Type II males.

Table 2: Indicators of Reproductive Strategy Number of Children Age at First Intercourse Number of Pregnancies Reported having STD*

Type

Mean

Std. Deviation

t

P-value

I

.34

9.54

3.7

<.01

II

.20

12.5

I

14.3 (years)

2.9

3.6

<.01

II

14.9 (years)

2.7

I

.64

1.3

2.6

<.01

II

.47

1.0

I

.14

.4

2.2

<.05

II

.08

.3

* Significant for males only, N=,759, for all other analyses N for Cluster 1= 583 N for Cluster 2 = 616, df for all t-test = 1197

Conclusions and Implications Consistent with the BSD model, it should be possible to identify two clusters of children, those characterized by harsh and unpredictable early childhood ecologies and those with more stable and supportive environments. As a result of this difference in larger ecology, the children should also experience differential parenting. These assumptions were well supported by the current analysis and two clusters of children were categorized into Type I and Type II clusters. As expected, the children in the Type I cluster experienced, on average, more change and harshness in their ecology and harsher, more insensitive parenting in the first 5 to 7 years of life. As an indicator of criterion validity children who had been previously identified as “high risk” based on behavioral and family measures (independent of LH theory or the BSD model) were over represented in the Type I cluster.

In accordance with life history theory, individuals growing up in adverse family environments may have reliably increased their reproductive success by accelerating physical maturation and beginning sexual activity and reproduction at an earlier age relative to those growing up in more harmonious circumstances. In this context, sexually maturing early may have increased the probability of having at least some offspring that survived to reproduce. In contrast, individuals growing up in stable and supportive family environments may have reliably increased their reproductive success by delaying reproduction and devoting more resources to social and somatic development. Consistent with this perspective, participants in the Type I cluster, with a childhood characterized by unpredictability and harshness, on average, engaged in earlier sexual activity than children in the Type II cluster. They also experienced more teenage pregnancies and consequently more children by age 19. Males were also more likely to report having contracted a sexually transmitted disease by age 19, indicating sexual risk taking. In summary, it appears that there is strong evidence that early ecological and family conditions specified by life history theory can be used to group children into meaningful clusters. Further, as predicted by the BSD model, factors of early childhood ecology appear to channel individuals toward divergent reproductive strategies later life.

Acknowledgements The authors would like to thank the members of the Fast Track Project for the generous access to their data as well as their ongoing assistance with this project. The authors also acknowledge funding from the Frances McClelland Institute for Children, Youth, and Families.


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