• links have (+)/(-) polarity, loops are positive (reinforcing)/negative (balancing)
  • S (same)/O (opposite) sometimes used instead of +/-
• stocks (accumulation of resources)/flows (rates of change that alter stocks) treated the same
• links must be causal rather than correlative
• loop is positive with even # of negative links, negative with odd # of negative links
  • polarity is known as the “open loop gain” of the loop
• indicate delays between nodes and their effects, are often responsible between short-/long-run effects