sibling SD

sibling SD.R

library(tidyverse)

#simulate datasets
sibs = 20
x = map_df(seq(0, 1, .01), function(r) {
  #simulate sibling scores at correlation r
  matrix = matrix(r, ncol = sibs, nrow = sibs)
  diag(matrix) = 1
  mvrnorm(n = 1e4,
          Sigma = matrix,
          mu = rep(0, sibs)
  ) %>%
    as_tibble() %>%
    mutate(
      r = r,
    )
}) %>%
  #change the scale to IQ norms
  {
    .[1:sibs] = .[1:sibs] * 15 + 100
    .
  }

#compute SDs
x_sds = plyr::alply(x, .margins = 1, function(dd) {
  dd[1:sibs] %>% sd()
}) %>% unlist()

sibling_SDs = tibble(
  r = x$r,
  SD = x_sds
) %>%
  group_by(r) %>%
  summarise(
    mean_SD = mean(SD)
  )

sibling_SDs %>%
  ggplot(aes(r, mean_SD)) +
  geom_line()