diff --git a/src/investment.rs b/src/investment.rs
index 1f5ed78..b483bec 100644
--- a/src/investment.rs
+++ b/src/investment.rs
@@ -147,6 +147,8 @@ mod investment_status_tests {
     use super::InvestmentStatus;
     use crate::types::PositiveFloat;
     use assert_float_eq::{afe_is_f64_near, afe_near_error_msg, assert_f64_near};
+    use fake::{Fake, Faker};
+    use rand::Rng;
     use rstest::rstest;
 
     #[rstest(
@@ -196,34 +198,70 @@ mod investment_status_tests {
             assert_f64_near!(status.net_profit(), net_profits[year]);
         }
     }
-}
 
-#[cfg(test)]
-mod test {
-    use super::{Investment, InvestmentStatus, PositiveFloat};
-    use crate::{AnnualContribution, Interest};
-    use assert_float_eq::{afe_is_f64_near, afe_near_error_msg, assert_f64_near};
-    use fake::{Fake, Faker};
+    #[derive(Clone, Debug)]
+    struct InterestRateFixture(pub f64);
 
-    impl quickcheck::Arbitrary for InvestmentStatus {
+    impl quickcheck::Arbitrary for InterestRateFixture {
+        fn arbitrary(_g: &mut quickcheck::Gen) -> Self {
+            let mut rng = rand::thread_rng();
+            Self(rng.gen_range(-2.0..2.0))
+        }
+    }
+    impl quickcheck::Arbitrary for PositiveFloat {
         fn arbitrary(_g: &mut quickcheck::Gen) -> Self {
             Faker.fake()
         }
     }
 
+    // Non-negativity: interest, gross profit, and net profit should never be negative.
     #[quickcheck_macros::quickcheck]
-    fn test_investment_status_interest(status: InvestmentStatus) -> bool {
-        status.interest() < status.balance
+    fn non_negativity(balance: PositiveFloat, interest_rate: InterestRateFixture) -> bool {
+        let status = InvestmentStatus::new(
+            0,
+            balance,
+            balance.0,
+            interest_rate.0.abs(),
+            PositiveFloat::try_from(0.2).unwrap(),
+        );
+        status.interest() >= 0.0 && status.gross_profit() >= 0.0 && status.net_profit() >= 0.0
     }
 
+    // Monotonicity: Increasing the balance or interest rate on positive years should not decrease any of the computed values.
     #[quickcheck_macros::quickcheck]
-    fn test_investment_status_gross_profit(status: InvestmentStatus) -> bool {
-        status.gross_profit() > status.interest()
-    }
-    #[quickcheck_macros::quickcheck]
-    fn test_investment_status_net_profit(status: InvestmentStatus) -> bool {
-        status.net_profit() <= status.gross_profit()
+    fn monotonicity(
+        initial_balance: PositiveFloat,
+        balance: PositiveFloat,
+        interest_rate: InterestRateFixture,
+    ) -> bool {
+        let status1 = InvestmentStatus::new(
+            0,
+            initial_balance,
+            balance.0,
+            interest_rate.0.abs(),
+            PositiveFloat::try_from(0.2).unwrap(),
+        );
+        let increased_balance = PositiveFloat::try_from(balance.0 * 1.1).unwrap();
+        let increased_interest_rate = interest_rate.0.abs();
+
+        let status2 = InvestmentStatus::new(
+            0,
+            initial_balance,
+            increased_balance.0,
+            increased_interest_rate,
+            PositiveFloat::try_from(0.2).unwrap(),
+        );
+        status2.interest() >= status1.interest()
+            && status2.gross_profit() >= status1.gross_profit()
+            && status2.net_profit() >= status1.net_profit()
     }
+}
+
+#[cfg(test)]
+mod test {
+    use super::{Investment, InvestmentStatus, PositiveFloat};
+    use crate::{AnnualContribution, Interest};
+    use assert_float_eq::{afe_is_f64_near, afe_near_error_msg, assert_f64_near};
 
     #[test]
     fn test_investment_simulation() {